Depends on a couple factors. Which ever one uses motion plus and has real time battle (if either of them do), I'll be more likely to get that one. It takes a lot more to sell me on an RPG with turn based battles, tends to get slow and repetitive.
Though as of now, I'm leaning slightly towards Last Story. I like the idea one of the original FF creators going back and trying to essentially recreate the series. Also, I'm far more familiar with the FF series than the Xeno games.