Nixy, from personal experince each day is great in its own way. The Sunday is when the hill climb finale happens, so there's a bit of a buzz, however the Sunday is always rammed solid, heaving, but great atmosphere. I would always lean to the Fri + Sat for that reason.
You'll still see loads of action, loads of amazing cars, bikes, racing drivers and grand prix / superbike legends, it will be amazing. A bit sad, but I went there for the weekend for my 'stag do', a bit more civilised, plenty of Pimms and 'Shampoo', it was great