What are the benefits of both? I've always kind of leaned towards 458, but ammo, when you can find it, is ridiculously expensive.
I'm looking at the 10.5" uppers for sale here
http://www.radicalfirearms.com/product-p/rfupper10.5-12.7x42-9fhr.htm
And here
http://www.radicalfirearms.com/product-p/rfupper10.5-458soc-9hfr.htm
Thanks

Can I assume you don't reload?  If you have thought about reloading I'd get the .458 and then use it as your reason to start reloading.  I did that with 6.5 Grendel.

if you don't reload find a 450 bushmaster upper and be done with it.  Its the only one of the 3 big bore's you can go into to a sporting goods store(IE gander mountain) and find ammo on the shelf.  Or you can order it for 24 bucks a box online.

if you don't reload find a 450 bushmaster upper and be done with it.  Its the only one of the 3 big bore's you can go into to a sporting goods store(IE gander mountain) and find ammo on the shelf.  Or you can order it for 24 bucks a box online.

I don't currently reload, but I have been considering it. I have a few 6.8 rifles that I could reload for.
The 450 Bushy never really appealed to me all that much. While I have seen it at the local Cabela's, Hornady is the only manufacturer that loads for it and they have what, 2 loads? That may have all changed since I last looked at them though.

These threads come up on a regular basis.  Which big bore AR15 is better??  Ballistics are almost identical.  Depending on who did the research, one might edge the others out but by a very slim margin, then on the next test another will be the winner.  If you are after ballistics flip a coin.  Easily found on the shelf?  I cannot find any of the 3 Big Bores on the shelves in my area at all.  There is a Cabela's 60 miles away but I would rather mail order it than drive that far.  

I reload and decided to go with the 458Socom.  It is a bit easier to reload and I have not had any difficulty finding bullets for it at my local stores.  300JHP, 325FTX 405FP and 300gn TTSX have been readily available.  Plus I keep an eye open on the 458 forum for people selling of their supply.  I have well over 1000 400gn and 2-3000 of the others mixed including some 500gn Pile Drivers.  Also have 1k of 305gn hardcast plinkers I picked up for $150 delivered.  If you are looking for a plinking round you can load them for less than 35 cents per round.  Then you can load the premiums like the Barnes TTSX for around $1.50/ round.  So everything else falls in between.  

Brass is the hardest thing to get.  I ordered 1-200 every time I found it available.  I now have 2k sitting on  the shelf and have been into this round for about 3 years now.  I currently own 3 458 SOCOMS.  It is my favorite round to shoot.  I don't go wild at the range but it is really fun when I go.  

You might be able to find factory rounds, it might be cheaper.  When I looked into the Big Bores I decided to go with the 458S.  It just appealed to me.  You need to pick the one that appeals to you.  If you do choose the SOCOM go with a Tromix barrel.  Great customer service and a great product too.  Oh, Tromix is also making 450 Bushmaster barrels too.

If you do go with a .458 SOCOM, for God's sake do not buy one of those knock-off chambered .458 SOCOMs sold by Radical, Wilson, Black Hole… anyone other than those using the original true to spec chamber and make darned sure you get a CAR or handgun gassed barrel (CAR for 14-18 inches and handgun gassed for less than 14 inches).  Midlengths will work with some powders but often not with powders that give the highest velocity to an already velocity challenged round.  To do otherwise you open yourself to problems.  Radical has had all sorts of problems with their .458 SOCOM barrels and so has Wilson.  This is well documented on the .458 SOCOM forums.  BHW is much better but any barrel using the PTG chamber reamer can run into trouble if using some factory loads.  If you do reload that is less of a problem but why buy a knock-off when you can buy the correct chamber for the same, or less money?  And that is not even considering the ethical question of companies using a knock-off chambering reamer.  Sadly, they are not doing anything illegal because the name .458 SOCOM was not protected, only the chamber dimensions.  
ONLY Teppo Jutsu, TROMIX, Rock River, SSK, and SBR make true spec chambered barrels.  ALL others and I mean ALL of the rest, are using PTG reverse engineered reamers or reamers of their own design but calling it the .458 SOCOM.  Some work better than others but you buy a knock-off chambered barrel then don't be surprised when you run into trouble.
If you do not know who TROMIX is, he is the guy who along with Teppo Jutsu developed the .458 SOCOM round and perfected its use in the AR platform.  SSK and Rock River are licensed to chamber the round using true to spec TJ chambers and SBR is doing work for TJ while the owner is out of the country for the next few years.
TROMIX offers barrel lengths from 8 to 16.25 inches for $299 including complete bolts (NOT BCG) with custom made extractors, not modified AR extractors.  All that really means is the TROMIX extractor gets more purchase on the rim for surer extraction.  There is nothing wrong with the modified extractors the others use but if a case sticks they are more likely to bend the rim or tear through the rim, but if you stick a case it is because of one of two reasons, you have thrown all common since out the window and tried to turn the .458 SOCOM into a .458 Win Mag or you are using a barrel with a reverse engineered chamber.

All this and more is found at the .458 SOCOM forums.  

Well Said!!!!

I have a .50 beowulf. Ammo is considerably less than .458,  it have been continuously available for the last 9 months.

So i didnt just buy .50 beowulf at Cabelas? Infact i believe the only one of the Big bores you can't buy in store is the 458 socom. Also i went with the .50 beowulf from radical due to the straight cartridge being easy to reload, as well as beig .50 which is pretty cool in itself, and not being 60-80.00 for 20 rounds.

After talking to radical, keep in mind the 10.5" 12.7 is like shooting a semi auto M44 Mosin. Recoil described as ridiculous.

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The big advantage to .458" SOCOM is that it uses standard mags w/ no modifications.  Just zeroed my brother's Rock River upper using a Cammenga steel mag, ran fine.

So i didnt just buy .50 beowulf at Cabelas? Infact i believe the only one of the Big bores you can't buy in store is the 458 socom. Also i went with the .50 beowulf from radical due to the straight cartridge being easy to reload, as well as beig .50 which is pretty cool in itself, and not being 60-80.00 for 20 rounds.

Cabelas is one of the few that carries it, at least in PA.  Here I have seen 450 bushmaster at several sporting goods stores(gander mountain, Dunham's sports) as well as a couple gun shops.  Prices are normally around 27 to 30 dollars, but I have seen it for as low as 24 dollars a box.  I haven't noticed 50 beowulf anywhere other than at cabela's which only has a  couple locations in the entire state of PA and MD.  I could have missed either because I don't own either so I only have noticed the ammo as a curiosity of whats available.  Around here it seems like everyone stocks hornady and Remington, hornady is for those who want a "premium" ammo and remington is for non discerning folk(i typically just use corelokts!)  If you want something other than those two you typically got to look a lot harder.  Walmart does stock winchester as well.