Anthony, 04-05 GSXR 600s and 750s look identical - the 750 just has the bigger motor.

GSXR600s are really good if you are going to race and might wad the bike. Parts are plentiful and cheap.

Honda's usually have the least amount of problems, and run smoothly for a very long time. They are a great street bike, and a very good track bike as well.

Either way, you can't go wrong. Go see Sung, he has a 07 CBR600 down at the shop.

Don't get the Duc. Maintenance is expensive on a 4 valve Duc, and parts are expensive if you lay it down. Start with something smaller, and you can always trade up later.