Answered: better traction ?

does the large omnidirectional wheels have more traction than the 4" diameter wheels ?

Yes it does.

oh ok … thanks!

I was thinking of adding some vex anti-slip mat to my 4’’ wheels to add more traction, but a friend told me that it could slow down my bot. Is this true ?

Increasing wheel traction does not directly decrease robot speed. However, if you attach it in such a way that it increases the “drag” on the robot, that could potentially slow it down. I bet it won’t be a problem.

I recommend you experiment to see what works and what doesn’t. Design is an iterative process, the more you play around and improve – the better your final design will be!