Maybe the best thing to consider is which one gets you on the seniority list the quickest.
I would have to agree with Guitar Guy on this piece of advice. Take the earliest class date possible, if able....unless you really have your mind set on one particular type of aircraft. Accepting a class date that is, say, one month later than the first one offered can potentially amount to a 6-month (or more) delay in upgrading down the road. It all depends on how big the classes are.
If given a job offer, I would also recommend telling the NJA that you would be willing to accept a class on short-notice, should they have a last minute cancellation (which occasionally happens).