I ride at both ADT and Encino (haven't ridden at San Diego in a while) and while the programs are a bit different, I don't think you can go wrong either way. They're both a lot of fun-- ADT is more structured (but still very friendly) and a nicer facility. It's for anybody, and they have programs for kids through whatever. There are even people who do the Tues/Thurs workout on spin bikes on the infield, and some of them eventually move onto the track. Encino is bumpy and can suffer from smog, but it's nice to ride outside, and there are really good racing and training programs. I don't know what's up at San Diego currently, but they generally have really good programs and a lot of active riders.
Encino is more of a grassroots program with some of the regulars getting more involved and running the various sessions. They have intro classes, open training, training races, keirin training, and regular races. Most of the non-race stuff is pretty laid back, and the racing is well organized and fun. You get more real racing practice at Encino, though that may change as they add racing at ADT.
ADT is more structured a lot of the time-- Roger has a couple of training sessions he runs Tues/Thurs nights that are as hard as you want them to be. They're typically a warmup, some intervals, a skill drill, and then a motorpacing game. They're good for both your skill and fitness. There are also open nights where you have to work your workout around other peoples, but it's usually not a problem.
As far as learning to ride the track, I can sort of comment, but I learned on another track and just took the super-mini class at ADT. I'd recommend that only if you've ridden a track a lot before-- there are a lot of etiquette things you'll miss if you just try to translate road experience to the track.
I suppose which one to choose depends on your level of experience and commitment-- Encino is less expensive, and probably more flexible and personal, but you might get a little less instruction on how to ride a crowded track or a fast race. If you're already obsessed with track riding, you may as well go for the good stuff and learn to ride the steep banking at ADT right away.
Roger puts a strong emphasis on safety and predictabilty at ADT - probably more than you'll get explicitly at Encino, with a lot of etiquette for riding a crowded track. He's a great resource for strategy and tactics of racing-- he just did a ~10 week racing class that was packed with riders all the way from noobs to former national champions.
My suggestion would be to try both-- I just got the updated schedule from ADT, and they've added a one-time "First Ride" deal where $15 will get you on the track to see whether you have to have more or if it freaks you out. Even if it freaks you out, it really is possibly as a noob to learn on that track. Lots of people learn for the first time on steep tracks, and most of them live to tell about it. It looks like the First Ride deal is 1pm saturdays, but it's probably a good idea to mail the contact email at the ADT web page to double check before you go. ADT Schedule
It looks like the next class at Encino is Aug 7th at noon, and will set you back $20, including bike rental. The guy teaching it is really good and nice guy, so it should be fun. I'm trying to get a friend who doesn't even do much road stuff to try it. Encino Schedule
Once you get past the initial learning cost, they can both be about the same cost per session. There are 10-session discount cards at ADT, and if you volunteer at major events (e.g. nats and worlds) you can get discounts too.