i think that your opinions might matter a bit more than ours! have you looked over the degree show archives for either school? i think that's quite a sincere indicator of the temperament of an art school. you don't have to like all of the work, but does it make you curious? and if you were to exhibit alongside it, do you think your work would be among friends? the slade and the ruskin have distinct flavours, but they're best described by what comes out of them.
more concrete differences exist between them too - living in london is a very different thing to living in oxford. i acknowledge that you're an international student, but if you could possibly attend open days (or just come and visit, i'm sure they would appreciate the travel difficulties and let you look around more-or-less whenever if you asked), that might be helpful. maybe consider: accommodation, living costs, class size, academic environment, city life, and how important any of those factors are to you.
i could talk about reputation/competition/prestige/success/X artists in Y gallery winning Z prize, but i think it'd be a shame to decide on that basis.
(i'm attending the ruskin next year + was a foundation student with the slade, so if there's anything specific you'd like to know then do ask :+))