Lots and lots of good info here:
www.m4carbine.net
For an all-around AR, I really like a 16" chrome lined barrel, midlength gas system, fixed front sight, flattop upper with flip-up rear sight (for ease of mounting an optic if you later choose to), adjustable stock. It's a versatile configuration that will do target shooting, 3-gun/USPSA, home defense, plinking, SHTF, whatever.
with the M/LE users of the AR world moving away from the direct impingement gas system - I've been looking at less expensive alternatives using piston-driven systems.
I don't see that happening; even SOCOM has decided to stick with the M4 instead of buying the SCAR-L, so I certainly don't see new piston 5.56x45's going to the general military. (The SCAR-H is another matter, but it's being issued as a DMR because it provides 7.62x51mm capability in a near-M4-sized package, not because of piston vs. DI issues.)
A properly set up DI AR is just as reliable as a piston AR; very, very few AR failures are the result of the DI system, as opposed to magazines or improper assembly.
Right now, I'm really curious about the Kel-tec SU-16C with the collapsable AR stock adapter. I've seen these at a combined price of around $780. I've had a Keltec P11 for 10yrs... and it's been amazing. I wonder what others know about the new SU16's.
The Kel-Tec SU-16 is a fine carbine, but it is not as reliable under hard use as a DI AR, piston or not. The Kel-Tec is designed first and foremost to be light and compact, and at that it absolutely excels; AFAIK there isn't a non-SBR .223 carbine on the market that stores more compactly than a folded Kel-Tec. If you want a light, easily transportable carbine that is reasonably weather resistant and won't be crawling through the mud with it or putting magazine after magazine through it, it is just about perfect, but from a durability and reliability standpoint the DI AR still edges it out, IMO.