Now, there's a broad question...

Personally, I'm a firm believer that you get what you pay for. Unless you're a gunsmith, I don't recommend the parts kits. My brother has one and although it was cheap, and was assembled by someone who'd put a couple together before, he's always had issues with it.

I own a Colt and a Rock River. Neither have any problems. The Rock River is newer and in spite of a 16" barrel, shoots right there with the 20" Colt.

Friends tend to recommend Bushmaster and DPMS quite often, though seems I've run into a couple DPMS on the range with strange spring problems and pins falling out. Easy fixes, but something you wouldn't expect to see.

It's a tossup as to whether I'd choose the 20" over the 16" barrel, all else being equal. Really depends on what you intend to do with it? While I'm comfortable with either (rifle or carbine), my wife is MUCH more comfortable with the collapsible stock and its inherent adjustable length of pull. Lots of guys seem to like the M4/CAR15 "look" - maybe it makes them looktougher or something?

Caliber? AR15 hashistorically been .223. I don't think anyone knows how well the 6.8 SPC, not to mention the .45-50 special-purpose cartridges are going to do commercially (ask someone who owns a 5mm Remington Rimfire how much they paid for the last box of ammo). Then there are the AR-10 clones, which can handle all of the .308 family (.243, .260, 7mm-08, .308, ...). And then there's the matter of magazine availability. If you want to hunt with it, you're going to be limited in most states to a 5-round magazine. AR15 (.223) magazines are WIDELY available in 5, 10, 20, and 30-round varieties for less than $20 apiece. Everything else I've seen for the 6.8 SPC, .45-50s, and the AR-10s have been twice that or more. So, though I can't hunt deer with it legally, I can still hunt turkeys and varmints - and I'd still choose the .223/5.56mm NATO.

Removable carry arm? I'm assuming that you're referring to the rear sight assembly and whether it be fixed or detachable/rail mounted? (and I hope your drill sergeant never hears you call it a carry handle, let alone sees you carry it that way). It's lots easier to mount a scope to the rail, rather than sit one way up on top of the fixed version. I'd take the detachable rear sights.