Endo, easily.
-Low overhead. You can get by with one front desk and one assistant. In ortho, you'l need a front desk, several assistants, and usually orthos have a case presenter/patient care coordinator.
-High production. Each procedure is at least $1,000. The endodontists I have shadowed can knock out molar endo in less than an hour. That means $1,000 per hour in production. Let's call it $800 just to be on the practical side. Working 32 hours per week, 48 weeks a year, at %50 overhead, that's $614,400.
Orthodontists are only paid for case starts, not for adjustments, which take up 99% of chair time.
-Hands-on. Most orthos hire assistants to place brackets, etc. While I get that some people prefer the treatment planning position, I would rather use my hands and spend all of my time in the operatory.
-Relief. Once the patient gets that RCT, they are instantly relieved of the unbearable pain they experienced beforehand. Relieving pain is a huge deal to a patient.
-Competition. There are A LOT of people going into ortho, and even more general dentists starting to do ortho. It is very easy to do Invisalign and 6 Month Smiles as a general dentist. This appeals to a lot of patients. However, not as many dentists are willing to spend 2 hours doing molar endo. Especially retreats.
Now, just to talk up the other side before people start calling me out.
-Most ortho patients will be more affluent, willing to pay cash for an "aesthetic" treatment. And since almost everyone is getting ortho nowadays, parents don't want their kids to be left out, so they will put up the money.
-Relationships with patients. You'll see the same families for 2 or so years. They'll talk about you with their family, you'll become a big part of their life.
For me, endo wins. I am more interested in it too. However, that doesn't mean I won't be using Invisalign or basic ortho as a general dentist.