Superman is soooooooo bad I don't have the time to even complete all my thoughts on it. To say that it falls apart in the third act is giving it too much credit; it becomes all but unwatchable. The only way to consider this a decent film is to view it strictly as an effort to show a Superman story from the 30s and 40s dropped into late 70s New York. Kent/Superman is overly hokey (going so far as to say he fights for truth, justice and the American way), Otis is stupidly over the top, Luthor's plot is really dumb, etc etc. I think it is not especially well directed (Amy and Devin were more than happy to shit on Donner in the review of Goonies though) and it is not even well written after the first half (Puzo actually has a lot of misses in screenplays).
The positives exist though. The relationship between Superman and Lois is very good and I like the scenes of Clark growing up in Kansas. It also IS a nice depiction of Golden Era Superman as I understand it.
Personally, I find Superman stories to be boring. The best works have the character much more world-weary; this is an alien with nearly limitless power stuck dealing with the problems of a race of people who can't get out of their own way. I know that would wear on me after a while.
Batman at least keeps my interest nearly 30 years later. I couldn't hate Tim Burton any more as a director than I do; I really believe he may be the worst director still getting big work that is meant to be taken seriously today. But I applaud many of his decisions in this film; it's a dark and dirty movie for a character that demands it. Batman views his surroundings this way. Batman is better paced as a film, better written, and has several more solid acting performances. It has its goofy moments to be sure (even as a kid, I thought the church tower at the end seemed stupid) but seems more palatable as they are presented than the overly sweet way as done in Superman.
Neither of these are really worth being in the Canon except for their place in history as comic book hero movies. If forced to chose between the two, it's an easy decision for me to pick Batman.