New and Balliol have some beautiful buildings and are way older. The atmosphere may be a bit different but I shouldn't worry about that too much. It might be more expensive as it's possible that you have to live out during your course. Balliol is academically quite strong and both have nice locations as well. But they are also more popular, so there's a chance (like 25%?) that you'll get pooled to another college (often less popular ones, like St Hilda's or St Hugh's) which may be quite disappointing (although some people might find these colleges fantastic), so as you are interested in applying to Somerville, you should do so!
Somerville might not be a medieval college, but has an interesting, unique history and is very pretty nonetheless (it's not 'modern' like St Anne's or St Catz but has this Victorian Era vibe besides modern architectural features). A pioneer in university education for women and maybe 'traditional Oxford' in that aspect. Somerville has an extremely friendly and laid back atmosphere, good hall food and one of the best libraries. The gardens are beautiful and you can walk on all grass. It's also one of only three colleges that provides on-site accommodation for all years and it has a really great location.
But remember it's all the same university in the end of the day so don't worry too much about colleges, you'll always love your college!