the toxic is just a better binned chip that will likely overclock slightly further than the dualx. that is of course as long as neither of the cards are voltage locked/limited while overclocking.
both the full gk104 and full tahiti chips are great gpus. its really a toss up and up to personal preference. nvidia has cuda acceleration and physx while amd has amazing gpgpu performance. neither of those mean very much for gaming though, physix is nice but steals performance, i turn it off. in the end the 280x is ever so slightly faster than the 770, though you would never notice it.