Roland,
The smaller the angle, the more detail you can get. I have 30, 45, 60 and 90 degree v bits in my arsenal. Smaller projects with a lot of detail will usually get a 30 degree bit, average size signs get a 60 degree mostly, and big projects, or signs with large text will get a 60 or 90 degree bit.
Depth of cut for a v carve project is calculated by how deep the bit has to go to fit between the lines, unless you set a flat depth. A 90 degree bit will cut much shallower than a smaller bit on the same project