It depends on your budget. Doing a drop in allows you to use tile for the deck surface. An undermount will need a solid surface of some type to undermount the tub to, and is consequently more expensive. As far as leaks, neither is suitable for a tub shower situation, which is the origin of the majority of leaks in a bath. Any other potential for leaks is taken care of by a knowledgeable and correct installation.

If you have a robust budget, you might consider a freestanding tub. A freestanding pedestal tub can be one of the best looking of modern tub ideas. It does require enough negative space around it to appreciate it, so isn't as suitable to a small bath.