Generally speaking they are use for different tasks.
The Belt sand is an aggressive sander used to level and even out surfaces. Even with finer paper in them they can tend to gouge a surface a bit.
The orbital is more of a finish sander and easier to use. It is very time consuming to try and level or reshape a surface with it. Even thought it can be done.
Make sure it is a random orbital or you will have circles everywhere!
If I had to do without one I probably would leave my belt sander behind. Unless I had to resurface or reshape allot. By the way most rental shops rent drum sanders for floors if they need work.