Last fall we opted out of a split stay due to the way the discount we used worked, and I think it was a bit of a mistake. The plus side of staying on Disney property the entire time was that we were able to shift our second Universal day to a different day of the week than we'd originally planned, since it wasn't like we'd save or lose any money by it. The downside was that we spent enough money on cab fare that I can't imagine it would have cost us any more to arrange a split stay, and it was a surprisingly long ride each way. If you're driving that probably changes things, but my preference has been to avoid driving when in Orlando, and I'm sure Universal charges for parking just like all the other parks do. I didn't realize Universal hotel guests got the express pass, which would have been nice.
Also, if you opt to drive in rather than do the split stay, do yourself a favor and bring a full change of clothes on your Universal day(s). We didn't on our first day, and we had a bit of a miserable afternoon (and a miserable cab ride) after getting soaked through to the underwear on the water rides.