Yip, generally mtb saddles are wider and more padded than road saddles. I've known some folk however to swing both ways (ooer) and fit a mtb saddle to a road bike and visa versa because they find it more comfortable.
The reason why a road saddle would differ from an atb one would be the rider position is not the same on both bikes.
Assuming the rider has both bikes and they fit him properly and they are set up correctly for his style of riding then you still have to consider how & where you ride, padded shorts or not, etc .
If you are starting out its hard to know what to go for, so see if you can try some of your riding mates to see what suits.
Some retailers do offer a 'try before you buy' on top of the range saddles. Consider this, a decent saddle will cost you the price of 2 good tyres & tubes (£70) and last a lot longer with luck . So dont skimp.