I think a divertor on the tub spout will end up being cheaper. Maybe not in the long term, though. You generally can't repair a tub spout, and when the divertor goes, you replace the spout. Most of these aren't that great a quality, so the cost isn't too much, but it's generally a throw-away part, may cost more in the long term.
A valve can be repaired.
WIth most tub spouts, when the supply is turned off, it falls back to tub spout (but not always!) and drains the riser pipe. A valve remains where you leave it regardless. This can be disconcerting if you are standing in the tub and planning to adjust the temp at the spout and it spits cold water at you from the spray. With a divertor, if you want to drain teh water out of the riser, you have to leave the valve mid-way after you shut the supply off, or you'll have that column of cold water there further giving you a start when you turn it back on.
So, part of it is what you are used to...either works.