If you are interested in a career as an academic after your PhD then you need to go to the uni where you can be supervised by the best researchers in your field. To be honest... it is very unlikely (except in rare circumstances or particularly niche fields) that the best academics will be in a uni that is 90-100 in the league tables.
Academics at top20 unis may have their time stretched, but at PhD level you should be working independently anyway, with guidance yes.. but not hand holding. Being thrown into an environment where you supervisor has more than 8 PhD students on top of MSc project students and undergrads (like i have been) means that you really appreciate the time you get with your supervisor. It really teaches you to be an independent researcher - to make decision by yourself which you then have to justify to the supervisor rather than saying "what should i do here?" and having them make the hard choices for you.
A friend of mine has gone to a slightly lesser uni where she is the only PhD student her supervisor looks after. She sees him for about 2 hours a week. I see mine for about an hour a month (if that) - out most productive meetings are where we bump into each other in the cafe and have lunch. Yes it can be stressful not having lots of time with your supervisor, but... it's making me grow as a researcher, more so than my friend who has hours with her sup. I asked her "why are you doing xx" and the response was "well, because my sup advised me to"... i can justify every decision i have made in my PhD so far with literature because i have had to make the decisions and present them to my sup myself - often via email which means i have to be specific and detailed rather than vague and hand wavy.
PhDs are tough, especially at top unis where you will inevitably have less time with your sup as they are busy people (i would say my case is somewhat extreme - most get about 2 hours a fortnight rather than an hour a month!). But, if it wasn't for their publication record and high profile i wouldn't have met other academics and developed links with researchers at other institutions interested in my work. Working with this sup gives my work credibility even though it is in a slightly "out there" area. If Prof XX thinks it's worthwhile, then there must be something in it!