In terms of cities, I find Liverpool to be a more friendly city with a better vibe. Manchester is great and there's a lot to do, but I find it quite depressing sometimes. Could partially be because it rains so much. For nightlife, Manchester is way better (WHP, Hidden). Liverpool still has great clubs but not quite the same level and not quite as consistently good. Liverpool does have a great bar scene though if you're more into that and less into djs.
To be honest, socially they're both great and you can't go wrong with either. But for me the music/dj scene in Manchester is better so that's why I'd favour it.