What should I get the 20 or the 18?  Is there that big of a difference?  It will mainly be used for coyote, hogs, and target shooting.  Also, what is the difference between the Rock River EOP and the regular flat-top?  Thanks.

I handled those at the last Indy 1500 gun show. The 20" is awful heavy. I wouldn't get over 18" myself but that's just me. The EOP is for large objective scopes.

There is about .5 lb of difference I think, but the 20" gives you a little extra velocity; I'm guessing around 100 fps more, which gives you a little more range and power.  Stick with the flat top so you can leave your options open on mounting accesories, don't bother with the EOP.  

I'd probably get a 18" if I was lugging the rifle around a lot out doors through brush or something for hunting, and I'd get the 20" if I was doing more target shooting.  I actually have a 20" Varminter which I only use at the range and love very much, even though it shoots better than I do.

UTPA  I have just recently received a 16" rra varmint with the flat-top.  Ive only shot it a few times at close range(50yds) with iron sights and am waiting on some scope mounts before I comment on accuracy. I purchased it for some of the same reasons as yourself.  Coyotes, hogs, plinking, and another gun for the house and bangin' around are a few.  The 16" barrel creates plenty of muzzle weight,if not too much.  Id hate to have a 20".  If velocity and downrange energy were a large concern, I wouldn't be shooting a 223. Ive shot a custom xp100 with a 12" barrel some and figure its enough for my needs.  Same with 16".  No matter how you dress it up(223), or lengthen the tube, its still a med range varmint caliber.  Unless you are expecting this thing to wear a scope every minute of its existance, I would skip the eop. just my $.02  

so do you think a 16 inch barrel would work for my uses?

Man yes, a 16" will work nicely.  I also agree with the recommendation to stick with a flat-top.  You can always add a riser block, or something like Bushmaster's twin riser blocks, but once you get the EOP, your stuck with it.  I would think that a 16" barrel is a good compromise for an all around rifle.  Short enought to carry in the brush, and you can pop the scope off if need be.  But excellent accuracy for the longer type work you might want to do.  

FWIW
Doc

The 16 inch will be fine, it will still shoot better than most can hold it and if its to far to shoot with a 16 inch barrel then its too far to shoot with a 223. I use the 16 inch barrel on yotes and its soooo much easier to manuever that short barrel around.

My target AR is an Armalite M15-A4(T) and IIRC it has a 24-inch bbl.  Very long and quite muzzle heavy.  So muzzle heavy I put a  3# lead weight in the butt to balance it out.  Man, that is one heavy rifle now but my God it says put when you shoot.  Too much to carry while hunting for sure but the weight is sure handy on the target range.  You shoot and the rifle just sits there.  For what I use it for, I like the longer barrel for the increased velocity and weight, but I sure would not want to carry it far.  Regarding the upper, IMO go with the flat-top A3 or A4 instead of the raised.  I hold a 6-20 Springfield in Warne Ultra High QD steel Maxima rings and it gives perfect alignment with a good solid cheek weld.  Those QD rings even hold a 500 yard zero if you choose to take it off and put it back on for whatever reason.  I had to take off my scope the other day and honestly, while I have had great luck with Warne rings holding a 100-200 yard zero, I did not expect them to hold a 500 yard zero.  They did and I am impressed.