I think Kennedy would be more likely on this issue. Roberts is a statist, so will generally side with the government (which would mean Prop 8 and DOMA upheld), while Kennedy has more of a libertarian bent (which meant he voted to overturn ObamaCare).
Kennedy has always voted for equal protection. Gay marriage will be the law of the land by end of June. 5-4 or 6-3. No way of stopping it.