Would you recommend external or internal ptosis repair? (Photo)

I have not personally assessed you.  There is no substitute for that.  No matter which approach is used, the examination needs to assess your degree of levator function. That is a measure of the excursion of the upper eyelid. That has profound implications for whether any ptosis surgery will be effective.  The posterior approach surgery is fabulously popular among eyelid surgeons for ptosis.  So much so, that many eyelid surgeons do not have anterior levator ptosis skills. However, there is a big, unspoken problem with the posterior ptosis surgery: the failure rate.  If you study the literature, the failure rate is vastly under reported. I can't tell you how many lectures I have attended where the speaker is talking about the Mullerectomy and when they show the after pictures, about half the patients are under corrected.  I have studied this issue anatomically.  There is a simple reason for this high failure rate: this surgery is okay for 1-2 mm of ptosis provided that the levator aponeurosis (tendon) correctly inserts onto the tarsus.  The rub is that the levator aponeurosis is often disinserted. That means that mechanically it is not in the right place for the surgery to work.  It is obvious from your photo, that your left levator is disinserted.  The Mullerectomy will not work for that eyelid.  I suspect that the right levator is also disinserted but the quality of the photographs you have posted are so poor that I can't assess the anatomic details on that side.  Don't talk yourself into the wrong surgery. Find an experienced ptosis surgeon.

you are correct that in general, Muellerectomy (internal approach) is for mild ptosis and external approach is better for moderate ptosis. That is the book answer. At 25 years of age, you likely do not have extra skin to remove so I would go with the internal (Muellerectomy) approach and avoid a scar. Probably the best way to decide is to see if a couple drops of ophthalmic phenylephrine can stimulate your Mueller's muscle to lift your ptosis to a level that is acceptable to you. Muellerectomy can only lift your eyelids a maximum of 2 mm, but that should be enough for you.