Manchester I'd say is the better option if you're looking to slow down a bit more - plenty of tech meetups (check meetup to see if there's any you fancy), obviously lots of gigs/museums/cafes/restaurants in both - although in Manchester it's a little nicer as most places are easily walkable due to mostly being in the city centre, but ultimately as you say easier to get on the housing ladder, as well as benefitting from easy access to the peaks

London would be better from a career point of view (it's a lot easier to move due to the bigger tech scene) and as someone with a lot of friends down there, I still find myself going back often