This is like asking me which hand I'd prefer to have chopped off. I need 'em both, but one is more integral in the long run and has to be kept. I'm going with T2. Why? Well, first of all, the dramatic beats of the film work well. Next, the writing is solid (especially in terms of the logistics of the Terminators and their little intricacies). Production-wise? T2 was a landmark in the visual medium and didn't get the recognition and praise for the ground it broke FX-wise for nothing. Jim Cameron was at his best.
As for Rocky II? Great film and one of Stallone's best efforts...ever. I still think that on the strength of the acting and writing (as well as the amazing fight scene) was actually superior to the original. However, in the grand scheme of things, I just think that the more original and trailblazing work was done with T2. Both films are tough cookies to compare because of the staunch differences in genre and tone, though.