Definitely do both, but the emphasis should be more on tactics puzzles than looking for tactics in master games. Studying master games is good for other educational value, as well as tactics, so it's definitely something you should also be doing. And playing lots of your own games, both at slow and fast time controls.

I have been pondering a question to which there is probably not a definitive answer:  Is it more effective to study tactics in isolation (i.e. tactics puzzle book, CT-ART, etc) or in context (studying master games in your openings, playing solitaire chess, etc).  Obviously, you get more bang for your buck time-wise by doing ten puzzles in a chess book but I wonder if tactics become more internalized if you slowly play over master games and your "spider sense" helps you to suddenly see the combination that is coming?  I'm sure the best answer is to do both, but I wonder on a fundamental level which actually helps you more.