Both the Beethoven Op. 90 and Mozart K310 are hard in their own ways. Mozart is difficult stylistically, technically, and musically. Every note is a gem. You miss a note in Liszt or Rachmaninoff, no one hears. You screw a note in Mozart, everyone hears. Beethoven Op. 90 is more Romantic, as this was Beethoven's first Late Period Sonata. Both of these pieces are wonderful, and in the end, it is up to YOU, to play what YOU feel most comfortable with.
Playing the hardest piece does not get you anywhere. If you play it poorly, it is much worse than being able to decently pull off a less-difficult work. Play easier pieces, but play them well. That is what is so hard about music-to play it well. Particularly the Mozart, which is technically and musically challenging.
Having studied both pieces, I would say both of them are very difficult, in their own ways. I will not compare, for they are two very different in the style and personalities.
All the best!