Tom:
We've cruised on the Sun, Legend and Sprirt. Like Sandra, the only real negative on the smaller ships is the absence of verandas. However, because the ships are so small, you are never very far from a deck chair to enjoy the sun and sea air.
The positives are the all inclusive bar, caviar without an extra charge, only 200 passengers - you really get to meet the other guests (if you want to). The crew does a wonderful job of making solo travelers welcome. Our friend that was traveling solo was invited to dine with different officers and cruise staff every evening. The entertainment is less formal, but often quite good. There's a small troupe doing shows, and often guest performers.
While they don't utilize it as often as some would like, they have a marina that lowers from the back of the ship to offer water sports while anchored in secluded bays.
Fewer dining venues,the main dining room, Veranda Cafe or in-suite dining (served course by course at dinner). There is nothing like Signatures, but I don't think you'd be disappointed with the food, it is usually very good.
The best part is never having to sign chits for drinks etc. and meeting very interesting passengers. The negatives are, no verandas (except in the large suites) and fewer activities.
Both lines do a great job of providing a upscale cruise experience.