I've played regular Resistance and I have Resistance Avalon, but I've never played Resistance Coup is it worth getting? I like that you can play with just 2 people but, besides that I don't know much about it.
I'd like to post an additional, though not entirely different answer to Joey's.
Coup is a completely different game.
Key distinguishing elements are:
Coup has no co-operative element like The Resistance does, it's entirely out for yourself and trying to outlive all the other players.
Coup is a very short game - games can be over in 30 seconds (for two people) to 10 minutes for a long game.
Coup can be played by 2-6 people.
In my opinion Coup is a very good addition to your board game collection. For its price and size, it's an easy game to add.
Basically - Coup is a very good warm up game. While everybody is getting settled, or you are waiting for people to arrive, you can play a few games of coup. The rules are nice and simple, most people understand them after one short game. Because the games are so short, it doesn't matter if someone arrives half way through, because they can just join the next game.
Coup is a distinctly different game. Whether it is "worth" getting depends on many subjective things. You might consider wording your question differently to ask about the differences to minimize the chance it gets closed for being too subjective.
In any case, coup is a bluffing game where you use any and all powers of 5 characters in the entire deck to increase your wealth and remove cards from your opponents. You can at any time use any character's power but can also be challenged (called out for lying) and end up losing a card yourself. It is essentially a bluffing and deduction game with no team aspect like Resistance has. I would suggest trying it to get a different experience from those of Resistance and Werewolf. You need to form your bluffing plan early and carefully work toward your endgame or you quickly end up in positions where someone is headed straight for a win unless you call him out, in which case he likely has the card he claims to have.