I went to my lgs to purchase a sbr and I was orginally looking for a 10.3 dd mk18 however the guys at the shop had a price cut on an 11.5 inch ddm4.  The price on the 11.5 inch was $1035 and the price on the mk18 is 1450.  I went ahead and went with the 11.5 inch because of price and the added benefits of the 11.5 inch barrel.  However now I am wondering if I choose the right one.  Is one inch really going to matter in terms of length when using the rifle.  Do the benefits of the extra length of the 11.5 out weigh the cons of the extra length?

Thanks in advance.

You did really good; you will be pleased with the purchase. But if you decide later you want the MK18 clone all you need to do is buy the upper.

I think you will really like the 11.5.  Don't worry about it being 1" longer, I think the better ballistics and slightly lessened concussion is worth the 1" length tradeoff.  I have one that is about 11" that I just finished and I'm wondering if I should have gone with 12.5"  And like he said, you can always buy the MK18 upper down the road and either keep or sell the 11.5"

You don't have issues mounting some silencers on the 11.5 like the 10.5 sometimes do.
I can't remember which brands but there isn't enough room in front of the front sight for some silencer mounts with a 10.5.

I strongly prefer my 11.5 to the 10.3's and 10.5's I've shot, including a mk18.  The extra inch makes no difference for me in terms of maneuverability and the slightly reduced blast and clearance for a can and my fingers is nice.

Just to give a different take.....Myself, I wouldnt trade my 10.3" or 10.5" uppers for anything longer.  Never had any issues with either.  The 10.3" I built, with the RIS II, and I mount an SDN6 on it with no issues....the trick is not to use a brake, you either use the comp or FH and youd be fine mounting the can.  And this issue is only when using an AAC can.  The 10.5" is an LMT, again with no issues.  

SO no issues, and a shorter barrel.....thats win for me.  Doesnt matter how its looked at, an inch is an inch, especially when mounting a can.  

I dont knock anyone for preferring the 11.5 or longer, but for me, its all about getting it as short as possible, while still proving to work.  

If you want to go for the CQBR clone...you'll need the 10.X anyways

You did really good.

You added 100 fps but lost an inch of maneuvering room.  Unless you bought it as a PDW you will be happy.  Even if you bought it as a PDW you will be happy.

I have both and would not part with either.

The Mk18 is a great gun with fantastic aesthetics.

My preference is the 11.5" though.  11.5's are a bit more forgiving if overgassed, and a bit more reliable if buying lower quality parts.  In theory, they should also be easier on suppressors, as you have less unburnt powder hitting the blast baffle - or brake if you run one.

This BCM post on 10.x vs 11.5's is a good read too.

http://www.ar15.com/forums/t_2_138/174764__Q__Why_BCM_chose_the_11_5_SBR_over_the_10_5_.html