I imagine getting post-docs for hep-ph is a little easier compared to hep-th, but still difficult. For hep-ph you also have the opportunities for working as a post-doc in nationals labs ( i.e Brookhaven , Fermi, SLAC, etc.) and as you have mentioned work is most likely to be directly related to experiment.
I personally have not met any post docs in hep-ph so I don't have specific examples or real life stories, but I am sure when you visit schools you can learn about the opportunities more. Now that I think about it, I have only met one hep-ph prof, so I don't know too much opportunities in this field.
If you are more interested in formal theory, do it. Work hard+find an advisor who will support you well! One caveat is that, as you know, getting tenure track positions in hep-th are extremely extremely difficult. The post docs I have met two years ago ( one was in his 2nd or 3rd, and the other was in 3rd) are still post docs. That's a lot of time...
One went to a top 5 school for grad while the other went to a top 30. Just to reiterate that it's not all about where you go....
Edit: Thanks for asking about my apps! I am pretty happy with the offers I got. I too wanted to do hep-th for a while, but changed my mind pretty much during the application process haha. I do still enjoy the field a lot tho. I find myself wanting to keep up as much as I can and talking to hep-th dudes/dudetts as much as I can.
