Different models have different features... the AP4 version is the 16" barrel with the regular looking receiver that includes the forward assist, has the standard A2 type front sight post, flash hider, etc.
The 308T is more of a Target model and is setup ready for a scope, has a gas block instead of a front sight post and has the "smooth side" upper receiver. Raised rail makes easier scope mounting. This is a 16" Heavy Barrel version.
There are various other versions including longer barrel lengths, but getting DPMS 308's has been difficult the past year... we get dribs and drabs... they don't usually last very long when they arrive.
There is of course another alternative and that's the Knights SR-25 but that's a whole other ball game... Knights recently released a new commerical offering for 2009 (the SR-25-LW) and brought the pricing down considerably from where it was before, but even with that reduction it's a $6,999.99 Canadian rifle (they used to be @ $10,000).
Mark