Classic for simplicity, storage capacity, and clean SQ output -VS- J3 for better & larger screened video playback, excellent EQ and sound enhancements, longer battery life, lighter & more pocket-friendly.
I have a Classic w/no EQ on and I use it with a Digizoid ZO - I'm happy with my choice. BUT,...I also use a Cowon D2, Sansa clip+ (RB), Sansa Fuze (RB) and Fuze+, Sansa clipzip, and a Sony Walkman S639 (plus others, I forget what I have sometimes,...).
All good quality players have (mostly) great SQ nowadays, so it's really about what features you desire the most. I owned a Cowon J3 - it's the ONLY player I regret selling! And, I've owned more than a few players,...
Also, the Classic sounds just fine using it straight out of the HP out - only someone who hates Apple will tell you otherwise.