They're consumables - just try them. If all else is the same, none of the jumps is going to be so bad that you'll want to toss them in the bin and go back immediately.

Having run 30s for a couple years, I do prefer 28s, but that's because a) the 28s on my wide rims are comfy enough and b) I like my bike to handle a little more to the agile/twitchy side. In terms of rolling resistance, there's no way that I could tell a difference.