It is a good question.
At a middling uni, where you are excelling, you could be freed from the stresses you'd face at a good uni. And it can be very stressful at times if you're not getting the grades you feel you deserve and need, but you're not sure how to improve.
But at the same time you might find yourself hating the lack of challenge, perhaps you'll struggle to mix with people who don't share your ambitions or intellectual abilities. On the one hand, you might have more fun because of less stress but you might have less fun because you struggle to assimilate with the intellectual defective.
I would say that having a very good degree (80%+) from a less established uni is better than having a 65% from a good uni. But life is not as simple as this; can you guarantee you'd be an 80%+ student?
Consider that you will be around people less motivated and capable than you; will you have the energies to push yourself to get that tippy-top grade? It is not uncommon for very bright people to go to mediocre unis and bomb. It is also not uncommon for people to bomb A-Levels, but excel at university. You are not guaranteed good uni grades simply because you were good at A-Levels.