The 6.5's are notorious for failing, especially the #8 cyclinder wall cracking...the 6.2's aren't noted for those problems. They are noted for longevity. If you want more power I wouldn't bother with a gas engine, put in a Cummins 4bt or 6bt. Or just call up Penisular Diesel and order their 6.5 (they use the newest IH/AMG blocks w/o the problems of the old blocks)...it is $6500 though.
So the Banks turbo is still cheaper, other companies make turbos as well. Someone told me to put a VGT turbo on the 6.2 for the best fuel mileage; but you are looking for power.