Don't forget that PPRuNe was conceived and is still run from Liverpool. The main difference is that whereas you could slag off Liverpool as BHTSM/NN has done, all you will get is humour in response. Do that anywhere else, Manchester or the Home Counties where BHTSM is probably from and you will get indignation and bruised egos.
As a Scouser, and proud of it, I would of course recommend Liverpool. Most of the points have been stated already. The ideal choice is, as mentioned, somewhere like Northwich which is just about equidistant from both Liverpool and Manchester, about 40 minutes each, as well as only 15 minutes from Chester. If you had to choose one city over the other though, I'd be bound to advise on Liverpool. Having been based at both Manchester and Liverpool airports, the hassle factor at Manchester is bad and getting worse. Even if you live close by Manchester airport, if you have to drive to the airport during rush hour it can be very frustrating and the staff car park is huge, not very well maintained and the bus service regular but a pain, especially when the weather is bad.
Depending on where your crew room is, you may have some distance to walk once you are off the staff bus and again, you will have long distances to walk from crew room to aircraft. They claim to have dedicated security channels for staff but more often than not, they are so understaffed that they put SLF through and you end up in a big queue.
It's been a while since I have flown out of Liverpool but the new airport there is impressive and you don't have far to go from anywhere when there. Hassle factor... probably a lot less than MAN.
As for the cities, Manchester is bigger and has more variety but Liverpool is changing rapidly. I hardly recognise the city centre anymore. Lots of new development and infrastructure improvements. Liverpool was chose as European City of Culture for a reason and there is a lot of choice. Stereotyping by southern woofters like BHTSM is just a sign that he really doesn't know what he's talking about. Go and get based down south for LGW or LHR and see how your quality of life ends up.
Just don't get confused about the difference between Scousers and Scallies. I'm a Scouser but not a Scallie. Unfortunately, some of the hoy polloi from down south aren't too bright when it comes to differentiating. Us Scousers regard them as 'Nice but Dim Tims'!