The acceptance rates are higher for several reasons.
Less people apply ED than RD and the ED pool is pretty much equally as qualified as the RD students. Therefore, Penn accepts many of the qualified ED applicants because they know that Penn is their first choice. Also, by accepting many during the ED round, Penn increases their yield since 99% of the ED acceptances will attend Penn. This increases their rankings and of course they want that. Recruited athletes and legacy are also taken into consideration during the ED round, so these people have a slight advantage for acceptance.
Not all, but many of the students accepted ED would probably get in regular. If you're not a legacy, recruited athlete, or another ED hook, your chances are probably not significantly better ED.
If Penn is not your first choice, you should not apply ED. If MIT is really where you want to go, go for it! Either way, I'm sure it'll all work out. Good luck!