I'm finishing 5th Pathway in June and have matched in Ob/Gyn.
First, the 5PW vs titulo route: 5PW is the better option because you get back into the US system a year earlier and get US experience. That's what program directors want from any foreign grad -- US experience. Yes, it helps that you have lots of hands-on experience from Mexico, but it's different when you get back. Lots of "defensive medicine," and tests that need not be ordered (at UAG they say -- and it's true IMHO -- that tests are for confirmation of a diagnosis made clinically, not for making the diagnosis), but in the US we order some tests to CYA later, just in case someone sues.
If you do 5PW, it's 5 years (4 in Mexico + 5PW) instead of 6 in Mexico.
Anyway, there's another option to getting the titulo: to get an MD from the NYS Board of Regents, you finish 5PW and apply for licensure from the state, and they will give you an MD degree.
Now as far as the competitive specialties... This year, no one from 5PW got radiology. Very few got surgery, and of the ones who did (that I know of), no one got a categorical spot, only preliminary. Some got Ob. Most got IM, FP, psych.
So if you have your heart set on radiology,
(1) study your butt off for the Boards, starting from day 1 at UAG, and get great scores; take Step 2 either in 8th semester or in the first few months of 5PW so you'll have the scores when you interview for residency -- and likewise you'll need to do great on it,
(2) maybe you can volunteer in Mexico and/or on vacations in the US with a radiologist who will give you a good recommendation (unfortunately, it's only the US letters of rec that "count" in a program director's eyes),
(3) try to do your 4th year "electives" in the US to get US experience before you get to 5PW,
(4) do 5PW, not internado+servicio social, and
(5) use your 5PW electives to do radiology rotations at places where you might want to apply for residency. You're more likely to get something at a community hospital than a university program, so aim high, but don't be disappointed. Also, you need to research your possible residency programs before you apply to see if they currently have foreign grads in their program or have graduated any in the last few years. One of the places I wanted to apply didn't have any IMGs since 1990 (the earliest listing), so I knew that it would be a waste of time and money to even apply.
Finally, just in case rads doesn't materialize for you, start making a backup plan.
Hope this response wasn't too depressing. Let me know if I can be of more "help."
Diana