510 is a nice mouse...... its also discontinued, or replaced by 610, a wireless laser mouse. The 518 is basicly the same mouse as the 510, but its uglier(sorry 518 lovers) and has a little more software. 51x series were made for people with big hands, so if you have small hands you might find it a bit cumbersome. I love it but I have huge hands, I could even use a size or 2 bigger. You should really goto a store and feel these in your hand before buying online.
Logitech G5 Laser
BTW I'm not suggesting the store, only giving you an idea of what these look like.