Thanks to people answering this same question for me before i bought my bolt - on: here's the scoop.
depending on what type of bike you have, you will either have a slip on or a bolt on. most newer sportbikes (japanese) are bolt ons; meaning, the new can bolts directly to the pipe. Slip ons (i think) involve a gasket or something.
i ordered my D&D from parts411.com; it came and I had it installed in less than 5 minutes.
If you're just after some sound and looks, go with a can. They all make about the same difference in HP and torque gains. Although, i've heard that slip ons and bolt ons are merely cosmetic. Fine with me. my bike had enough power to begin with. it just made it sound sweet.
scope out some other riders and their exhausts and see which you like best. after i bought my D&D, I heard a Two Brothers pipe, and thought for my next can on my next bike, I'll go with the Two Bros.
Basically, go with looks, and something in your budget. I think i got my can for $232 or so. parts411.com saved me about $50 vs. buying from D&D's website.
Hope this helps. Good luck.