I'm going to offer a different answer than seems to be the theme here. Unless you find the prospect of living in Puerto Rico for four years exciting (which you might), I would strongly consider the DO program. My reasoning is as follows:
1. You're suggesting that Ponce is going to cost you a lot more money than the DO school. Unless you're using the military scholarship program or have some other way of avoiding student loans, you should take every opportunity possible to minimize your debt burden. It will effectively reduce your take-home pay significantly for at least a decade after you're done with residency and/or fellowship.
2. Emergency medicine is of moderate competitiveness, and assuming you come in with good medical school grades and board scores, I don't think going to Ponce versus a reputable DO program will dramatically impact your chances of matching into emergency medicine either way.
3. If you're not fluent in Spanish, I would imagine the clinical rotations may be more difficult, as many of your patients are going to be exclusively Spanish-speaking. Again, you may be bilingual, or you may view this is as a great opportunity to learn Spanish.
Finally, the caveats to the above advice would be:
1. If the DO school is of questionable merit, is new, or is on or has recently been on probation...then my advice would lean more towards Ponce.
2. If you love the idea of going to Puerto Rico for the adventure, then have at it.
3. If you envision yourself being a big-wig researcher or academic admin in the field of emergency medicine, then you may want to choose the MD program.
But in general, if the DO program is significantly (say 20% or greater) cheaper and reputable AND your main goal (of which your fairly certain) is to be a community ER doc...then I wouldn't let the mere fact that the other school is a DO program push you too much in favor of Ponce.