It's true Manchester is a bigger city, but only in terms of it's outlying towns and the overall region. The city centre of Manchester is actually very small by usual city standards. You can walk from one side of Manchester city centre to the other in about 25/30 minutes. The university is about 20 minutes walk to the city centre, but the university location has a very campus-y feel to it and Manchester doesn't have lots of huge skyscrapers, so you never really feel like you're in a city until you walk around the centre.
It's also easy to reach and has the UK's second biggest airport. 2 hours from London on the Virgin train network and the university itself has a great international ranking. There's about 14,000 overseas students there too, so it's nice and diverse and the majority of people from the UK will tell you (if they have visited) that Manchester is a really friendly city.
To tell you the truth I do not think you would regret coming to either, but as you can tell I think Manchester will be the better fit.