Better IMO but others swear by 6. Still more say 9 is the best.
I would just try it for yourself. They're both quality games regardless of which one is better. :)
If you're looking for specifics of why I preferred 7, here are a few:
1. The aesthetic. FF7 has a sick dieselpunk style that's less common than the steampunk feel of 6. 2. The worldbuilding (and story). There's tons of lore in both games but 7's lore feels grander in scope to me. 3. 3D graphics. Not THE biggest thing but FF7 was one game where 3D graphics made a huge difference. Granted they've aged terribly but it allows more explicitly shown scale to lots of things and more impressive summon animations. 4. Breadth. I counted the other day and there are at least 29 minigames by the criteria I considered, and the majority of them are a part of the story or terrain traversal. Keeps things fresh.
Now there are also things I would definitely say 6 did better, including:
1. RPG elements. In 7, the characters are largely interchangable, aside from limit breaks (limited use special moves) and a couple of stat points. This does allow more flexibility, but it's IMO it's not a favorable tradeoff. 2. Dynamic world. 7 does change a bit a couple times but nothing so pronounced as in 6.