DB's Matrix - Easy to create and use. Can be made to look quite nice with careful programming and texturing. Built-in commands for collision. Marginally slower if used conservatively, but a lot slower if lots of large matrices used simultaneously. Ideal for beginners to learn with.
Heightmap - isn't actually a terrain, its simply data which is used to set the heights of all the points on a matrix (or object being used for the terrain).
3DS or .X - Harder to create. Can look really stunning. You have to program your own collision as if it was any other object in your game. Faster than DB's native matrices. Needs more advanced programming knowledge and not ideal for complete novices.
For a
simple terrain, a normal matrix would do fine - it's up to you.
TDK_Man