Hi Cecily~
I've played both the piano and the harp (I'm currently still learning the piano actually~) and both instruments are great to play! However, from experience I have to say that playing the harp is way easier than playing the piano (however I gave up harp since I didn't have time). Although at first, you might find it uncomfortable, you'll get used to it in no time as your hands will develop and get accustomed to plucking the strings so it'll be fine and there isn't that much technique required to learning the harp whereas the piano, there's loads of technique and touches that need to be learnt to be able to play it well. So IMO, I'd say go for the harp and plus it sounds really really nice~
Also if you want to play the harp, my family is currently leasing our harp~ (since my bro and me don't have the time to play anymore), Although it's a baby harp, it should suit you fine seeing as you'll be a beginner to start off with and you won't require the grand harps as of yet~ Hope this post helped and pm me for more details~