If you believe the 3/4 MOA accuracy guarantee, I would like to interest you in some beachfront property with a bridge to boot. If you do not produce 3/4 MOA groups, it will be your problem not RRA's because they can do it any time, I'm sure.
RRA is a good brand, I happen to be more inclined to ArmaLite, others will mention various other vendors.
The good thing about a complete rifle is that you get a warranty.
As for your question, the answer is simple 20 inch is the optimum barrel length if you want to carry the rifle around and shoot it without a bipod. If you want longer reach, 24 inch is the way to go.
What is a slighter bbl?
If you reload and can't get to .75 moa with a Wilson tube in an RRA gun it would be the first I"ve ever seen that wouldn't do it. I have seen 2 that needed to be recrowned.. at one point it seems like their "crowning" person used a 3/8 drill bit... but I have not seen that issue in a number of years. Thats .75 moa at 100 yards. There is a difference at longer ranges. Whether you need more or not is up to you.
On the Armalite, at times I"ve seen them not function as a complete rifle out of the box so each vendor can have its own issues at times.
You can't really go wrong with the normal builders. Weight is an issue up to you, weight really doesn't have much to do with accuracy bascially.
Barrel length is also personal, personally I dont' see anything over 24 inches being a big bonus. But you can loose a lot of speed under 20...
