The EBJD variant comes riddled with health problems. You can pay alot of money and still get a poor quality fish that just doesn't grow. I've seen many that grow up looking deformed and many other that seem to not grow at all. Mattlee's current EBJD seems to be growing exceptionally slow, and there are one or two others on the forum also. I havn't seen or heard of anyone in a long while who had one who grew anywhere near the size mine was, and look half as good. Which shows you how hard they are to come by, especially in good health. I lost mine the other week at only the age of three, one morning he was just dead. It a gamble you take with them i'm afraid. IMHO it was worth it with mine, he was something special and well worth it.
In general they will grow a little smaller than your average JD, mine seemed to have topped out at just under 9", but they are slightly slower growers than standard Jacks, so he could have grown some more but slowly over time. Termperament wise, mine was a big baby. The only fish he ever chased was his 7" female, i kept him with tetras, firemouths, rotkeil sevs, bichirs and plecs and he was absolutely fine to them all. Generally they are "thought" to be less aggressive, and this is a sentiment i somewhat agree with. Obviously they are a cichlid at the end of the day, so the odd one may be a little more boistrious then the next, so again its down to the fish you pick.
Overall, if you can find a good blood line and pay something you deem appropriate for one they are truely magnificent fish. I've heard a few people throw out the compliment that they are possibly one of the most 'marine' looking tropical fish you can get. Quite a compliment for a fish which nobody truely knows if its naturally occurring or man-made.
Good luck with your decision.
James