I agree with a lot of things said in this topic so far, going to try not to parrot too much but it's inevitable.
I think FF4 is the better game, for a few reasons, although I'm hardly a huge fan of either.
FF4 is just fundamentally much more playable to me. I've completed the game four times despite my opinion of the game, but FF8 only once. Why? I even first played them around the same time (early in my RPgaming career when I tended to replay things). Obviously FF4 is shorter, that accounts for some of the difference. But the bigger issue is that the game flows in a far better way. FF4 does not waste time on shit like mid-battle drawing or managing chains of item creation to get the resources you need. FF4 does not waste time with long sections of plot (both games have shitty plot and awful female leads, but FF4 scenes never last more than 3-5 minutes tops I'm pretty sure, ending aside). FF4 battles flow more smoothly and are generally more balanced in terms of having a good challenge level; the game has some decent fights in there, instead of FF8's model of "this fight will be unpredictably mediocre to awful based on your junctions".
FF8's heart is in the right place in many cases. It's certainly a more ambitious game than FF4. Squall is a much bolder and more provocative character than anything FF4 can dream of. But FF8 is certainly the less enjoyable game to play. It pisses away some of its good ideas, in both writing and system, with bad execution of both, while FF4 is able to successfully be that light romp that provides enough of a challenge to be engaging. I sorta feel FF4 is what many Dragon Quest games aim for; simple and playable with nothing majorly offensive even if there's a lack of innovation in there (the fact that FF4 has a worthwhile main character and more interesting battle design than most DQs is just a bonus), which to me is superior to FF8's general mess, although I can see respecting ideas more than execution enough to flip this.