'Should you wish' is more formal than 'if you wish, at least in American English. We don't use 'should' in this phrase much. I'm not sure about British English.

"Should" has the subtle connotation of something which may happen in the future. Realistically, "if you want/like" would be most common "if you wish" is fine English and sounds very nice, so maybe in specific instances and "should you wish" probably only in a book or in an educated conversation where the specific "should" needs to be used.

None much. The first is more natural sounding. The second is “fancy” persay. Hope it helps.