Keep the risk of static electricity in mind. Wheels in general on robots are Vann Der Graff Generators with the frames storing charges like Leyden Jars.
Omni Wheels: It is a wheel with 10 more wheels on it. This adds up to 11 VDG Generators. Add two Omni Wheels and you get 21 VDG Generators. Multiply that doubled up Omni Wheels by four, one on each corner, and you get get 84 VDG Generators. Boggling risk for static.
Mecanum Wheels: There is still a risk as there is with the Omni Wheels. There are two Mecanum styles I've seen, Stationary such as this
http://cdn3.volusion.com/vyfsn.knvgw...jpg?1410248327 and non-fixed rollers
http://cdn3.volusion.com/vyfsn.knvgw...jpg?1410248327 Stationary rollers are lower static risk and non-fixed are higher risk.
All of this is based on what I've seen. More wheels/rollers, greater static risk. That's my 2 Cents.