What do you think you'd primarily be using it for? And what kind of budget are you thinking? I'll give you a quick run down of both and desirable features at the end, but some of those may not fit your budget. If all else fails get a chrome lined 1:7" twist barrel with a 5.56mm chamber and m4 feedramps though. The ramps should be well cut and line up with the cuts in the upper receiver as well.
The direct impingement system is a more proven design than the piston on the AR platform. A good quality gun of either type should serve you well though. I own both and am in the process of building another DI in left handed flavor for my wife.
That said the DI guns are generally lighter and have a softer recoil impulse (not a real big deal though, were talking about the recoil of a .22 caliber firearm). The DI is also a more tried and proven design with a greater availability of parts and can generally be had for cheaper and in more configurations.
The piston systems are easier to clean, but generally heavier and take proprietary parts. Most parts will be the same except the gas system (front sight base, gas piston & op rod/spring vs tube, gas key on the bolt carrier or sometimes the carrier itself). All the lower parts except maybe the buffer are the same either way. The piston guns tend to cost more though.
In either case I'd recommend looking for manufacturers that use magnetic particle inspections and high pressure tests on the barrel and bolt carrier, a properly staked gas key (piston guns may not have a gas key at all, but part of the carrier machined to take the from the from the operating rod). Also try to find manufacturers that have chrome lined barrels, 1:7" or 1:8" twist, a 5.56mm chamber, m4 feed ramps, and a pinned front sight base. Some may say you need mil-b-11595e steel for the barrel but 4150 or some other chromoly is fine.
The piston uses a spring. On a DI gun there is a steel tube that runs from the front sight base to the receiver, it dumps gas into the gas key on the bolt carrier and pushes everything back into the buffer tube. When the carrier comes forward again it strips another round off the magazine, and we do it all again. A piston gun does away with the gas tube and instead uses a piston connected to an operating rod. This rod has a spring on it to return the piston back to it original position after the rod strikes the bolt carrier and sends it back into the buffer tube. The 2 systems just vary in the mechanism they use to send the bolt carrier backwards, on e it's back everything's the same.
Between the 2 brands you list I'd get bushmaster, just because I've had one. Depending on your budget and use I may suggest others though. There are to s of solid manufacturers out there these days, we' be got it good.
