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It really depends on what kind of game you want to run. If you're just getting it to read, definitely pick up Cthulhu. I personally, would much rather play Cthulhu than Star Wars any day, but both only as (multiple) one-shots; I simply can't sustain enough interest in Star Wars for a full-blown campaign and the PCs don't survive long enough for a full campaign in Cthulhu!
To recap: I like horror a lot, so I would go with Cthulhu.
That's a strange question. They are so totally different that it really depends on what you WANT more. I think rules-wise SWRCRB is much better honed, but d20 CoC is still playable and probably a better bargain if you are looking to add stuff to a normal D&D game.
Choose Cthulu. The reason is WoTC has stated that they may not reprint that book. But, Star Wars they will. In which case, you want to get the book while available. If I would have known how good it was, I would have bidded on the signed copy of it..