To the best of my knowledge, the Prime (UA21A) only uses the Intel HD 4000 integrated graphic core. The UX32VD comes equipped with a nVidia GT 620m. Other major difference is the size of the laptop; the Prime has a 11.6" screen and the UX32VD has a 13.3" screen. The larger screen makes things easier to read, but also increases the weight and increases power consumption which reduces battery life. Lastly, the Prime can only be equipped with up to a 256GB SSD while the UX32VD comes with a 500GB hard drive with 24GB SSD cache.
Obviously, the UX32VD will provide better gaming results because of the GT 620m, but it is a somewhat weak card. I have seen some game benchmarks where the Intel can actually beat it's little brother the GT 610m. In fact, based on those results the Intel HD 4000 is actually better than the GT 610m. The GT620m is faster than the Intel HD 4000 and the GT 610m, but not by very much
The best? Not sure. The screen is certainly nice (and you'll pay for that) The UX32 is larger (basically twice the thickness-- 18mm vs 9mm), has more ports but ultimately the same innards. . based on that one would think the UX32 would have less thermal issues just based on more roominess for air to move about (think U36G vs U31SD).
Whichever is cheaper.
There is something to be said to see what the T430u is going to be, but I don't think Lenovo is going to be spec-ing 1080p screens any time soon, but I'm somewhat excited about it and the starting price is around $800 or so.