I think the Spirit class ships have 80% ocean view rooms so chances of being upgraded to an oceanview room if booking an inside room would be greater. A lot of people think this ship class is just the right size.
The Conquest class is a newer, larger ship. I guess it just depends on what you like. I've never been on either class of ships. Just the Holiday and next month I'll be cruising on the Fantasy. I hope to try the other classes on my next cruise.
I guess you can't go wrong either way.