Well I bought the aluminum because they look closest to stock in theory while still having those awesome tips. I'm pretty sure performance isn't different so it's more preference.
Make sure you participate in the group buy if you get tit. or cf. The forum group buy does not apply to alum.
I just ordered TB-TI from Admin (Group Buy) for $693.91 (US Dollars). Thats w/o dB killers, with the FE Kit, shipping is included and also a fee that Admin has to pay. I think thats a pretty darn good deal...
In short, I don't think there is a 'good reason' why to spend a few hundred more for CF/Ti cans. It's a personal choice/preference.
My 'reasoning'...I'm already spending $600+ on an aftermarket exhaust so what's another $200+. I waitied/saved for over 2 years to get these and had my mind made up on getting the CF cans.
It's the same 'reasoning' as when you buy a new car. You could save yourself some money and get the 'base model', or spend a bit more and get the 'upgrade package'. Sure, you don't need it to be able to drive the car, but since you are already spending this much, what's a bit more?
Man, they made a huge difference for me. My friends can now hear their bikes when we ride. I still like being able to blip the throttle and have drivers notice me.