The S&Ws are pretty good for the money. I'd much rather have one of them than a carbon 15. Just so you know, most Bushmasters aren't corbon/plastic, just the carbon 15 is. That is their newest entry level gun. The stags are also good for the money. If I were looking for an entry level carbine, I don't know what I'd do, there are a ton of good options.
Rifle vs carbine is berrel and gas system length. A traditional M-16 style rifle will have a 20" barrel and a rifle lengtg gas system. A carbine is more styled off of an M4 and most often has a 16" barrel and carbine length gas system.