Despite what the name suggests, omni wheel actually have more traction than what traction wheel has. However, whether to use them or not also depends on some other factors
4in wheel - The stock 4” traction wheel sold by vex has an OD smaller than the omni wheel. When placed in the middle, it will not touch the ground and therefore can’t act as a center wheel. Using locked omni works great for center wheel however. However, another alternative is to wrap the 4” traction tire around a 30T sprocket - typically called a 169 traction.
Despite having less traction than locked omni, it has the added benefit of having a larger OD than omni wheels. With a 6 wheel drive, having a center wheel with larger OD will “drop center” the drive. This can improve turning speed of the drive. When only 4 wheels (half the drive) is in contact with the ground due to the larger center wheel, the wheelbase length is effectively halved while the wheelbase width stay the same, reducing the turning scrub and therefore increasing the turn speed. However, there probably won’t be a difference if the robot is heavy enough that all 6 wheels can contact the ground due to the wheels sinking into the tiles.
3.25in wheel - I don’t think the omni is easily lockable as the rollers are smaller (may still be possible using 6-32 screws). Usually i just see teams do traction wheels in the center as the OD is close enough that it touches the ground.
2.75” wheel - Not many teams actually use 2.75” wheels for their drivetrains, but I’ll suppose that they are similar to 4”wheels - lockable, can 169 traction by using a smaller sprocket to wrap the 2.75” traction wheel with.