The two seem to complement each other well - grind stumps during hot summer, spade trees in spring and fall. I've know a couple of guys who do just that and are very successfull at it. You will, however, need to know a lot about tree biology in order to know when, how to properly move different species of trees.
Most the spade operators I know will guarantee for 1 year the trees they sell but there is no way to warranty a tree that they are simply moving from point A to B for the customer - might not be able to find an equal replacement.
Prices for services depends on location of your business along with size of spade used and distance tree(s) have to be moved. also, if you mulch or water in the trees as part of your service, that has to be added on.
Biggest problem I see with starting up in the business is that, if you don't have your own inventory of trees to sell/move, you're going to have a tough time of it. The big jobs tend to be in property development - spading in numerous if not hundreds of new trees onto a bare property. the individual jobs of 1 tree here and 2 trees there are likely more difficult to make money at just as with stump jobs due to travel time, bid time, etc etc. Do you know of any local spade operators who could use some help with the idea of a buyout down the road?