It's not a black or white answer. There are many variables. Where do you currently play/live(taxes, cost of living). If I live in Texas and get an offer in NY or Toronto, that 20% increase in pay doesn't really amount to much. Now is it's something like the Sox offered 100M(just for math sake) and Seattle offered 150M? Then I could say the difference is large enough to take the risk of leaving for a new company/team.
The company/team is also a consideration, am I leaving Nike to go work for Dexter? Or am I leaving Dexter and going to Nike? Am I actually happy where I am or am I looking for my first chance out?
There is a bunch of stuff that is taken into consideration that only the athlete and agent are privy to. It's really a tough subject for us to judge from the outside.