Whether it's a true Pernil cut (from hind leg), or a shoulder Pernil cut, it should work fine for pulled pork.
I prefer to trim the fat cap down to about 1/4", and cook fat side down so the meat side develops a better bark, but that's personal preference.
Boston Butt versus Picnic cuts: Over the years I've cooked a lot of pulled pork for some large crowds (Marching band cookouts, banquets, graduation parties, etc.). In a nutshell, if you specifically want the money muscle for slicing, then yes, go with a Boston Butt. Otherwise both are shoulder cuts and will work equally well for pulled pork.
As far as yield: The Picnic has more bone, and the Boston Butt has more of the fat cap (which ends up being trimmed off), so it's your choice.
Some years ago, at one of the Marching Band BBQ's, I did a blind taste test of Butts versus Picnics with all the adult staff & volunteers. There was no clear winner because no one could tell which was which.
For large cooks I just get whole shoulders and have them cut in half (separating the butt from the picnic), because it's more economical. At home I just use whichever cut I have.
This was four whole shoulders I got from the butcher, with the shank removed, and cut in half (i.e. four Butts and four Picnics).
Hope this makes sense!
