Sounds like you are doing about what I did. My first AR was a Bushmaster Dissipator (complete rifle). It works great and I'm very happy with it (although I wish a fluted barrel had been available to cut a little weight).
Next, I built a kit on a Busmaster stripped lower. I got a 20" stainless bull barrel on a flattop reciever with a free-float aluminum tube handguard. I assume the second rifle is more accurate, although I've only shot it with cheap South African ammo. I get groups of about 1"-3" at 100 yards. I'm sure if I added a better trigger, used better ammo, and WERE A BETTER SHOT, the groups would shrink. The rifle sure LOOKS accurate
I originally ordered the 20" thinking that it wouldn't be too different from a standard AR-15 rifle to carry around, but it is heavy.
I can't really compare the Dissipator, since I don't have a scope on it, and my eyesight and ability don't allow for such precise shooting at that range with iron sights.
If you have a complete lower, I'd probably order an upper from JT or Sherluk to put on it. If your lower is stripped, get the whole kit. If you want real accuracy, get a match-grade trigger. I did have some problems getting mine to work - even had to send it back, but now it seems fine. I'm sure a Bushmaster upper would be fine (and probably less risky), but if you like the stainless, and/or want to save some money, try the other suppliers.
Check
http://www.ar15.com for lots more info and opinions.
Doug