Basically ENTJs are obsessive planners and ENTPs are obsessively anti-plan.
In a social setting it can be hard to tell the difference. We're both condescending jerks that have no problem calling you out on anything we perceive as unintelligent or incorrect, we love to prove our point and we love to show off our mental skills and abilities to everyone.
Moreso than other J/P dichotomies, ENTPs are distinguishable from ENTJs because we're constantly making it up as we go along. We can get into a heated argument fairly easily but we'll drop it and swap sides or completely reverse positions once the next interesting/fun task comes up.
Interest in following through is a huge difference. ENTJs are a lot more methodical. They're typically concerned with productivity and efficient use of time, which ENTPs don't give two shits about. ENTP would rather go off on an Ne rant creating all kinds of new and amusing ideas and entertaining people in the process, then discard that idea as soon as it gets boring.
ENTJs make better group leaders. They're harder workers in most cases, because the only time ENTPs are hard workers is when the "work" is actually enjoyable or otherwise interesting--if it's not, expect nothing more than the bare minimum contribution, along with an occasional sarcastic complaint about how uninteresting the task is.
I think the best way to sum it up is that ENTPs are interested in idea creation and ENTJs are interested in actively putting those ideas to good use. For ENTPs, once the idea has been created and served its purpose by providing a brief escape from boredom, it's hardly of any real use or importance anymore. For ENTJs, forming the idea and the plan is only the beginning, a precursor to accomplishing some broader strategic goal.
ENTJs are also more insistent upon their specific conceptions of how abstract theory should be viewed and applied. Both types will go off on rants about the "proper" way to view their pet theories/interests, but ENTPs are doing it to break the mold/generate entertainment for themselves and others, and actually aren't that dogmatic about adhering strictly to these conceptions. When ENTJs do it, they're genuinely pissed off most times, because theoretical incorrectness is both irritating and offensive.
There's this famous ENTJ poker player, Phil Hellmuth, for instance, who gets seriously angry when he sees people play hands poorly and get lucky to win anyway. He's very outwardly vocal about correcting the theoretical mistakes of others--ENTPs do this too, but mostly just to show off their own knowledge and abilities. Part of us wants to win, but part of us is also hoping you'll come back with an intelligent response so that the exchange can continue providing food for thought. We revel in ambiguity; the tactics involved in the exchange itself tend to take precedence over the actual content of the exchange.
Not so for ENTJs: when they get deeply into an argument, they are on a personal crusade against inefficiency.
For another example, take standup comedians George Carlin (ENTP) and Bill Hicks (ENTJ.) They have similar angry-rant styles, but Carlin's motives are harder to discern. ENTPs' motives have a reputation for being difficult to read, because they're so busy improvising by the seat of their pants that half the time they're not even sure what their own motives are. They love to swap sides in arguments just to challenge themselves and their own analytical abilities.
With ENTJs, there's no time to fuck around with devil's advocate or saying things you don't mean. You know where they stand. Everyone knows where they stand, because they make it known forcefully. They give a much stronger, "Don't trifle with me" vibe.
ENTPs, on the other hand, enjoy trifling and being trifled with. That's part of the fun! Where will this trifling lead? We don't know, and we don't care, as long as it's not boring.