Depends on whether you have space. I would pick the vise if I had only room for one.
The shave horse excels when you want to rough out a stave into a bow, up to floor tillering. I prefer it to using a vise at this stage because its easier to clamp, manipulate the stave, and take off large amounts of waste.
For finer work, when the limbs are flexing a bench and a vice gives you better control, and you can work the bow when it is strung.
Have both if you can.
