Bottom Line: you can't go wrong with either ship. I've cruised multiple times on all except for the Dream (only once).
If you go on the Wonder you will have a lot fewer first time ever cruisers, and not wanting to be rude, but the first timers usually come in with the "I've got to do everything because I won't ever be returning" mentality, that for me, makes the Dream way to crazy & hectic with lots of people that "don't know the drill".
So being that you are 3 adults, the Wonder should be MUCH calmer. And I like calm. But DO NOT mistake "calm" with "boring, nothing to do" because the opposite is very true. There are a lot of things to do on all of the ships. Very enjoyable.
Another reason for the Wonder with 3 adults (assuming 1 stateroom) is that the rooms are enough larger on the Classic Ships vs the New Ships that it Does Make A Difference.
I would go for at least an outside room (porthole or verandah) so that you have a room with the split bathroom - it really helps with the getting ready process (for the day or for the evening).