Personally in terms of grass starters I always would pick bulbasaur, treecko, or snivy. Never was a fan of turtwig or oshawott and Snivy is a good pokemon in my opinion. He was my starter for White 2, White, and Volt White but between Mudkip and Oshawott, I'd go with Mudkip but since you have to battle Fern and Flornia who use grass type pokemon, Mudkip will put you at a disadvantage and might get one shotted by something like razor leaf or giga drain. Then again mudkip will be useful in the long run against the poison gym leaders.