Each of the two universities is well regarded. The University of Warwick is close to Coventry, which is a rather underwhelming place (except for the Basil Spence Cathedral), but students often live in Leamington when not living on the university campus, and can go clubbing in Birmingham if they want to. Nottingham is a big student city with some Robin Hood-related tourism. There's little to choose between the two, so, as suggested, go and visit both if possible.