If this is the only difference, go to the top university.
The reason is that what is "exciting" to you right now can very easily turn out to not be exciting once you start working on it. An example from last century is the search for Planet X. This hypothetical planet beyond Neptune had been predicted from theory, and now "all that's left" is to actually find it. Given that Neptune had been found a few decades ago from similar theoretical considerations, this can very easily sound exciting. After all there were at the time only 8 known planets in the Solar System. Finding the ninth would permanently etch your name into history. If you find it, you would also be looking at something nobody has ever done before, treading new ground, writing new chapters in humanity's knowledge from a blank slate.
But what if I were to tell you that finding new planets involves looking at millions of otherwise-unremarkable images of the sky using a blink comparator? It's boring, manual work that still has to be done because there's no better way to do it, and you need to do it for years.
Something similar could easily apply to your case. The end goal might be exciting, but the process to get there is what you'll be dealing with every day, and that might not be exciting. It's not easy to predict from the outside (or even from the inside) what the process will be like - a modern example might be you discover that your calculations work for simple cases but for the more interesting cases they are too slow, so you need to find ways to improve the speed. If you dislike figuring out how to make code more efficient, then you are not going to like the process even though the end result is still exciting.
This neglects the other advantages of going to the top university: more/better colleagues, visitors, library access, brand name for post-PhD job searches, etc.
This doesn't mean never go to the lesser-known university, but you need more/better reasons before doing so, e.g. they offer better funding, your supervisor there is well-known even though they're based at a lesser-known university, your current supervisors recommend you go there, etc.