[Kevin Leonard]

Everyone here is posting this wishy-washy stuff about how it depends on your strategy and that mecanums could have uses, but the OP asked "which do you think is better", and whenever asked that question in a game with defense, I'm going to respond with traction wheels.

Sure, you can absolutely build an extremely competitive robot that does every task in the game with mecanums. It is possible. But if you don't have experience with mecanum (and even if you do), traction is going to be simpler, better at pushing, and is easier to make traverse obstacles(8" pneumatic wheels will get over obstacles easier than 8" mecanum wheels when controlling for other variables).

Both have advantages, but unless there's some really specific use case where mecanum is the overpowering option (like it may have been in 2015), or you have tremendous experience and driver practice using mecanum, go with traction wheels.