I'd put them both on the GPU - that way they both get the benefit of its performance.
Unless it's a very old GPU, there aren't really any drawbacks to having both of them on it. (My old GTX460 won't go down to its lowest power state with two monitors so it idles about 15°C hotter than with one monitor but that's not a showstopper. Newer cards don't have this problem. The last time I wasn't able to run two monitors off one GPU was with a Radeon 7000 - the original pre-X-series one, not an HD7xxx - where it simply didn't have enough video RAM.)