The only S2's worth mentioning are the Epic Touch and the t-mobile variant. For whatever reason all the Skyrockets I have tried have some noticeable lag. (I blame this on the snapdragon processor since I have yet to a handset with one that didn't stutter.)
If you really want the S2, get the Epic Touch, as its not only the fastest tested phone thus far, it comes with the original intended Exynos chipset, which absolutely screams. Its a damn shame the gnex didn't come with them, but from what I've been reading, certain chips don't play well with LTE. All in all, I will say the Eptouch is the best phone in the world currently. (HD screen aside) A real travesty that it cannot be flashed to verizon, otherwise I wouldn't own a nexus.
If you have grandfathered unlimited, the choice is obivous, stay with VZW and stick it to the man. Also, FWIW, running roms on a gnex not only clears its problems, but significantly improves its performance.