Did RSEA offer you funding? I think a lot depends on that -- if you have to take out loans to do it, definitely don't go. If you got funding from Harvard, press Penn for an answer about funding.
I said this before, but, while I think RSEA is a great program for a lot of reasons, if you're already sure you want to go on to a PhD and you have a competitive PhD offer, I'd definitely recommend going straight to PhD. There is a lot to be said for gaining the contacts of an MA at a different institution and the experience of writing an MA thesis, so I don't know, maybe this is just me, but I'm exhausted of applying to schools, of moving every year, of getting older without seemingly getting any closer to a settled career -- I only felt this somewhat before doing the MA (and I didn't apply to PhDs during my MA cycle since I was switching time periods) but now I feel it very strongly. Doing an MA might make you a better scholar in the long run and I don't regret it, but there's a lot of anxiety and precarity involved in my experience.
If you really want to work with Harvard's premodern China people, it's possible to do year-long (or semester-long?) visits among the Ivy League if you're at an Ivy League school (with permission from advisors etc of course.) I don't know the details but I've heard of people doing them!