This is an interesting comparison since both games are the ones with a 'love story' as part of the central theme. FFIX is the other one, but it blows both of these out of the water so let's ignore it.

I prefer FFX. It has some flaws with the voice acting, boring villains, and linearity, but I really liked the CTB and the sphere grid was a pretty cool idea the first time they used it (just got played out when XII and XIII/XIII-2 used it). Also Auron is cooler than any FFVIII character.

FFVIII has the GF's which were cool and it was a nice change of pace on how summons worked, but I found the characters overall pretty bland and drawing magic was the worst FF gameplay element since FFII.