I used to read them as a teenager, and enjoyed them when I was a big sf fan back in the day. Asimov's was the better of the two then, and featured monthly editorials by him (often hilarious), his book reviews, and the occasional new short story of his that debuted in the magazine. I haven't read it since his death, but I imagine the mag is somewhat different without his personal touch.
Another one I liked even better than those two was
Fantasy & Science Fiction. They seemed to attract a lot of talented upcoming sf writers, and Stephen King (!) also wrote an exclusive short story each year. Lesser-known but high quality fiction magazine (or at least it was in the early 1990's).
