I just ended up buying shorts as well, I was in some serious pain using my road bike and at I would have to stop or lift myself off the seat. The shorts do help a good deal, but as stated here, the right seat and angle are also important.
Funny enough I've had zero problems with my MTB, maybe because I don't spend as much time on the seat while on the trails as I'm usually up off the seat a lot. As to which type of shorts; for something that is going to be used a lot I prefer to spend money on something good that will last and be comfortable.