[Jizvonius]

Omni wheels will make turning much easier. The major disadvantage to that system is that while you can turn yourself with less effort, so can everyone else.

I would suggest only changing out either the two front or back wheels with omnis. This way you can still turn easily, but you can resist a little pushing from another robot. Note that your turning center will shift towards the non-omni wheels in this case.

The sprocket weight thing can be calculated based upon the size of your sprocket and the densities of the metals. If your current sprockets have large hubs, the weight savings could be significant

Edit: so a lot of this repeats Cody's post, but the sprocket calculations still stand.