I experienced the same situation. Between high-ranking university versus topic of research (with clearer lab/research information shown on website, but I don't know both professors of the both schools). My dilemma left me in mentally drained weeks Well, now decision is made, I can't make any changes, I got nothing to lose = if I can make it to get my degree, it's from a top-school anyway!
Btw just curious, why you regretted? The professor is an old/non-energetic one or less funding?
best