I can only give you my view as an applicant
I visited both, Mansfield on a study day and Magdalen on an open day
Personally, I felt Mansfield was quite pretty but nothing in comparison to Magdalen and it's stunning grounds.
I also wasn't too fond of the food at Mansfield (pretty disgusting actually), though I'm not sure if this is just because they didn't try because there were lots of Year 12s. I didn't eat at Magdalen so can't comment on that but the dining hall was prettier and the drinks were cheaper in the shop!
I think if I wanted something a little less intimidating id choose Mansfield, but personally I wasn't daunted by Magdalen and just felt it was the place id like to live in
I would say though, that I have two friend who visited Mansfield and loved it and both have decided to apply there! It really depends on what you're looking for!
If you can't visit (because for us three this is really what decided it) then I'd just suggest thinking carefully about whether you want a larger college, bigger grounds, or a small more community feel etc
Sorry if this still seems indecisive! I just don't want to seem biased because I've chosen Magdalen :P