It all depends on what you want. Do you want speed? Do you want comfort? Do you want something you can take you and the hunnie on for a few days? My suggestion considering you already have a quad and know how to drive something with a clutch, is to not go small at all. I guide a lot of people into getting a Honda rebel, Buell Blast, or a Ninja 250 for their first bike. But that is for people that haven't rode anything before, and usually for girls. If I had a little more info on what you are looking for, I could help guide you better. I have had bikes for 10 years now, and have rode numerous ones. I am pretty partial to Honda, but a lot of brands out there are decent. No matter what, I would stay away from Harley, unless you like lots of maintenance and some of the uncomfy rides for a cruiser. Also, what are you looking to spend, and how is your driving record and age? As a sport bike is fun, but can be pricey w/ insurance if you have lots of tickets or very young.
2008 700xx; Power Bomb header with power core 4 exhaust, Power Programmer III, pro armor nerfs, prm products control front and rear bumpers and full skid, 10" ITP rears, ITP Baja's fronts, RZR 2's all the way around, Uni filter, 13t sprocket, CCP steering Damper, DS450 shock swap, Dual Gasser Throttle, bar risers and Moose barkBusters. 730XX kit plus other engine mods!