Both have good and bad points...the catless will flow more and yes you may lose some backpressure. The catted will not flow quite as good although the hi-flow cat pipes are pretty close. Also with the catless one you will have alot more exhaust fumes in the car especially if you have headers and dumps on them. My old 91GT was like that and you could get out of it smelling like exhaust fumes. I now have a hi-flow MRT h with dumps and like them. Also of course the catless would not pass emissions if your state has them and the hi-flow has a better chance. The catless is of course cheaper than the catted one also..but technically it's "road legal" and the catless pipe isn't "road legal". That's my .2...
i have a bassani x, with bolt on cats, straight pipes. I have tried it both ways and i think you lose too much back pressure wtihout the cats because i feel like i killed my low end.