I'd go the R7 240. If both cards are the same price or close it's not a contest.
I'm going to build a new pc and I want to add 2 extra video and sound cards for two scart tv's in a multi seat setup.
I'm a bit hazy on some of this, but before you build a single PC have you worked out how you're going to split the focus and controls, and what to do about switchres? I'm sure a lot of these issues could be fixed by running some sort VM instances, but...
MAME and GM can only focus on one instance at once. If you try to run two instances of a driving game and network them only the one in focus will receive control inputs.
If you're running a dual-screen vs game in MAME, like a back-to-back fighting game, or Outrunners, you should be fine in this first regard, but you might have trouble with the video output, as switchres only works with one screen. Unless i've got that bit wrong, and because both screens are the same res, and running from the same instance of GM, it'll all work fine.
Other emulators like M2emu or teknoparrot, are probably going to be more forgiving about the first point, and shouldn't have issues with the latter. You might have problems getting them to open on the correct screen, but you can use something like Actualmultiplemonitors to move things around that if you have to.
Old PC LAN games like Grid or Outrun 2 C2C, and don't kid yourself that last is awesome, might not be possible as you won't get two instances to work cleanly, if at all. Again, instances in separate VM's might get around this. Might actually be better than two PC's, in fact.
So I guess the question is, what are you planning to run? Either way the best thing to do would be to build the single PC and test everything first, don't start running cables through walls or bolting a frame together until you have everything working.
EDIT: oh, and if you were thinking you can simply clone the displays i don't think you can, MAME won't let you. Certainly you didn't used to be able to. You could test this out as well, and if it fails, a SCART splitter would be a better bet.