I've never played Xenogears so I'm kinda biased as FFVIII was one of the greatest games I've ever played . I would just play both games from the start and see which one keeps me going. I did this when I purchased FFVII, VIII and IX on my PS3 and now have 90+ hours of FFIX and maybe 30mins in both FFVII and VIII.
I absolutely love FFVIII, and I dabbled in Xenogears. The most major annoyance to me in VIII was the magic-draw system but the idea of junctioning magic and GF's was a lot of fun. I'd reccomend VIII, but definitely come back to Xenogears after!