What is the difference between spry and WA validation toolkit

I've never used spry server side, so I can't comment on comparing the two.

Validation toolkit server validation is completely customizable, so you won't run into any limitations in what you can do other than what you might not be able to conceptualize anything is possible.

If you ask something specific I could tell you how it was done with webassist server validations. I generally use spry for client validations and validation toolkit for server validations. I might use a few client validation features from validation toolkit for really complex validations or reformatting options, but the bulk of the client validation work seems better done with spry these days.