Its a mixture of both you should be playing. Its impossible to let yourself not in a hand when you are getting something like 20-1 on a call, thats just playing stupid. When you play without statistics you are letting yourself away from some very basic math that can increase your stack without risking a lot of your chips. Psychology is obviously a big part of the game, but this only comes into play when youa re playing asgainst much better players, you cant play psychology against players that arent good or dont understand the game very well, and if you try you will just end up getting called or sucked out on. So make sure if you are playing the psychological part of the game you are playing against players that are good or understand the game. YOu cant bluff a bad player its just never going to happen. You must play statistics tho, if you have a 25% of winning the hand and its only 1-10th your stakc to call and you are getting 5-1 on your call... you have to make it! these are basic math statistics you can not ignore especially if you are playing large MTTs. This is where all the grunt work takes place and where all those books on odds
and such matter. Dont dismiss psychology though, it is a big part of the game especially when you get short handed and when you are playing against tougher more experienced players. They will know whos playing tight and aggressive and who they can bluff and who they cant. So take notes and keep a close eye on the players at the table and that should let u know where u can use psychology. Good luck and dont ever ignore the math in poker!.