If you afford the Banks kit, it has EVERYTHING you need, down to little bits of hardware and the exhaust system - there will be no parts store runs. It also performs very well on these engines and is of high quality. They'll put a dent in your bank account (unless you find a good used system like me).
With the 6.5, it'll involve some finagling to get everything going correctly. If you buy all the parts new it'll be more $$$ than a Banks kit, if you find em used you'll probably save some money.
As far as performance difference, Banks claims to outperform the 6.5. What they actually mean by this I'm not sure, but I do know that all compressors are not created equal. GM's stock exhaust system on the 6.5 also sucks.