I get and read all three, Rodale's Scuba Diving, Dive Training, and Alert Diver. The three are very differein from each other.
Overall Scuba Diving is most readable, with the best graphics, and most imformative travel articles. If you're in the market their product reviews might be useful, though I'd take them with a grain of salt.
Dive Training might be very useful for new divers, since they provide lots of what I classify as "fill in the blanks" info that's useful to inexperienced divers.
Alert Diver might provide the most in depth technical information for those who want to learn more than the basics, though it often can be tough reading.
Those are my opinions, but in the final analysis only you can decide if any of these are worth the dough. Scuba Diving is available at newstands or large bookstores, so you migh pick up a few issues before deciding whether to subscribe.