Several companies offer the early-entry tour of the Vatican Museums; I did mine through Walks of Italy. Typically these tours hurry through the museum hallways to the Sistine Chapel, which allows you to see it before it becomes a sea of shoulder-to-shoulder humanity (not that it will be empty even when you get there, but it won't be mobbed). Once the regular opening hours begin, the crowds increase throughout the Museums and especially in popular places like the Raphael Rooms (which were in near gridlock by the time we got there). We returned to the Sistine Chapel later for the purpose of exiting through a "back door" that tour groups are allowed to use as a shortcut to St. Peter's Basilica.
But the other part of the question is, "Do you want guided tours of these places, or just to gain entry?" The Vatican sells "open tours" of the Museums (which include the Sistine Chapel)--you get skip-the-line access plus an audio guide, and there are early-access options. They also offer their own guided tours which are cheaper than the tours from the commercial companies; I don't know how quality compares.
Entry to St. Peter's Basilica is free, though there are often lines waiting to get in, and unless you can slip through that back door at the Sistine Chapel, you have to walk around from the museum area (and it's not right next door; the Museums are HUGE).
I think the tour guide was worth having in order to efficiently locate some of the highlights of the Museums on a first-time visit. When I go back to the Museums some day, I'm planning to go on a Friday evening (when the Museums stay open late) and wander on my own. But I didn't feel like I needed a guide for the Basilica; there were plenty of other information sources offering similar information
