Congratulations on passing the rescue course, one of my favourites

I'd say go with the DM internship course, but don't rush to get the cert, take your time, enjoy it and make the most of it, and the free dives on offer!! (took me three and a half years to get through mine, but that's another story, and possibly a tad too long! ).

As AmtRosie says, the best teacher is experience, but this is what the internship course provides you with, and lots of. You'll be bolstering your diving knowledge with the academics, and gaining broad and general diving experience with the help you provide, both in the store and in the water. And of course, you don't have to take the next step to instructor afterwards.

One thing I would recommend, buy yourself a copy of the PADI Instructors Manual, make sure you are fully conversant with the standards, the tasks you are allowed to perform as a DM in Training. It's your own responsibility to ensure that you don't break PADI standards, you could be taken to task personally if you do.... it's not unknown for instructors to try and bend the rules a bit, if you do get a badun, their methods may also implicate you.