Examples of some dual sport bikes:
Kawasaki KLR650
Kawasaki Versys
Suzuki DR650SE
Suzuki DR400Z
Suzuki Vstrom 650
Honda XR650L
Honda CRF230L
These are the mainstream DS bikes. They are basically dirtbikes that are legal for street use...turn signals, brake lights, electric starters (great feature). Some are more street-oriented while some are more dirt-oriented. More street oriented (KLR, DR650, Versys) would mean larger fuel tanks for longer distance touring, street tires, luggage racks, windscreens, less suspension travel. XRs and DRZs are more suited for offroad riding (longer suspension, smaller tanks, knobbier tires, no windscreens) and not very comfortable for long highway miles, but for buzzing around town and daytrips into the countryside they're fine. Parts are cheap and readily available for all the mainstream japanese bikes.
They are relatively low tech (with the exception of some of the european makes such as BMW, KTM), but dead reliable and easy to work on. Some engines are water-cooled, some air-cooled. There are large aftermarket options available for most of the japanese bikes (luggage racks, hand warmers, windscreens, engine guards, more comfortable seats, etc). Some have very tall seat heights (XR650L), some low (DR650). Being able to touch the ground flatfoot is a reassuring feature to some, especially riders just starting out. Some people like the feel and handling of a lightweight bike over a heavier one.
I have an XR650L, and wouldn't take it on long distance trips anymore. After 100 miles or so on the street, I've had enough. The seat is uncomfortable and the buzzy engine noise and vibration is tiring. I recently rented a Honda Goldwing (a dedicated touring bike) and spent a day on it, and I really appreciated the extra storage space, comfortable seat and riding position, giant windscreen (there is no windblast hitting you) and smooth, quiet engine.
I would agree with the others and try to test ride the bikes first. Some dealers allow test rides, some don't. Where I live the city dealerships don't allow test rides, but the ones in more rural areas did. There is a subjective feel to each bike that will help with your decision.
I would pass on the new Yamaha VMAX as a first bike as well. It's a land-based fighter jet...