I don't have a wealth of experience with cruises, so far only three. Anything with 1,700 or less passengers would be ideal. For that reason, old or new really doesn't matter - to a point. That said, I like the light, open layouts of modern ships to the darker, traditional ones. I prefer old ships, but am very particular.
Mediums lack the ambiance of the small ships and the organization of the mega-ships.
I would not, however recommend my preferences outright to a first-time cruiser. With all the glamorous new ships out, I don't think a small ship is the right place to start. We have generally gone on a newer, larger ship each cruise. We like the relaxation at sea and the opportunity to see new places but the new, large ships are magnificent. There are lots of places to spend time and a variety of night spots if so inclined.