Went to both earlier this year. Each has its pluses and minuses. Big Bend NP has some spectacular stuff to see which is all well documented meaning there will be crowds. BBRSP is more remote and rugged with waaay less people. I'm guessing you and the missus are much like me and mine and there won't be any 7 mile/all day hikes. If that's the case, do both. We spent 4 nights in BBNP and felt we had seen what we came for without feeling rushed. We then spent another 2 nights wandering around the Terlingua area going to the SP and then 2 nights there. We could have easily spent another 2 days in the SP but we were running out of time. If you go, be sure to do Pinto Canyon road on the way back north. Its about an hour west of the SP exit and well worth seeing.