before you take my opinion , I should tell just a ballroom dance student who tried different types of dances before a found the dance that I'm passionate of (obviously ballroom), I'm not consider myself as a professional and not intended to be one.
If you learn one dance you can learn other. if decided to learn hip hop before ballroom you just need to work on your posture and your following skills. but you will catch-up a lot faster than anyone without dance experience. if you decided to learn hip hop after ballroom ( which I've tried to do that) you need to how to move differently but again you will catch-up a lot faster than anyone without dance experience.
which one is more difficult? it depends on individual differences. if you like both ballroom and hip hop and you have the time to do both, then go for it. if you can only choose one choose based on your priority.
Good luck.