I have done both and my view is
If you want a fast time then Liverpool is far better, Wilmslow has a very congested start and is a pain in the arse to get to the start/baggage area with all the cars double parked, also causes problems getting away after.
Liverpool is also the bigger event this year 5500 entries against 4000 (I think) for Wilmslow.
If you want scenic countryside then wilmslow wins