The main advantages of going to a formal postbacc program are the amount of support they usually provide (through advisors, tutors, etc), linkage programs, structure (having it all planned out for you), being surrounded by other postbaccs, and usually having a committee that will write you a letter of recommendation. Some of these programs have very high admittance rates for their graduates (Goucher, Columbia, Tufts). However, they tend to cost a lot.
I don't know of any statistics comparing postbaccs in formal programs to postbaccs 'doing it themselves'. In my opinion the students at formal programs will have a slight advantage just because of all of the opportunities and support they are given, but you have to weigh this against the cost of attending. I'm taking my courses at UCSD, even though I seriously considered some formal programs, because of both cost and location. As long as you're willing to work a little harder to seek out the advisors, tutors, fellow students, etc you should be fine at a 4 year U.