Personally, I really like the two classic ships. There aren't quite as many amenities, but the ratio of amenities to people is preferable. It's a more intimate setting, you get to recognize the other guests and crew, and it's easier to navigate the ship.
That said, I think it's unrealistic to presume they'll sail FOREVER. In a meeting with a senior staff member on a recent cruise, they suggested that it really comes down to the plumbing more than anything else. If there were a need for a significant plumbing overhaul, the cost in drydock would outweigh the value returned from the repairs.
Long story short - if you haven't been on the Magic or the Wonder, you might want to prioritize them.