Do you want to be a narrow specialist, or a wide ranging generalist?
When you're taking care of your patients, do you want to be responsible for them head to toe? Do you want to know about absolutely everything that's going on with them?
Or do you only want to be responsible for 1-2 body systems? Do you want to know an enormous amount about 2 systems, and a decent amount about how every other system connects back to the lungs?
Do you want to be a specialist? Or a generalist? Wide ranging responsibility or narrow? RN or RT?
I can't speak to how RN's view their patients, but I can tell you that as an RT, when I look at a patient it's an "all roads lead to Rome" kind of deal. Except its the lungs everything leads to and not Rome, as it were.
When I get a head trauma patient, I'm thinking about neurogenic shock. Stroke patients I'm investigating if there's midbrain/pons involvement. That guy in liver failure, I want to know what his albumin is. The lady with CKD - what's her BUN/Creatinine? Etc., Etc. We RT's tend to see every body system in a "how will this effect the lungs" kind of way.
As a nurse, you're less likely to be Sherlock Holmsing everything back to the pulmonary system, instead you take a much more general view, seeing all body systems working in unison. Which sounds more appealing to you?
As for opportunity? An ADN will give you more, because being a nurse gives you more opportunity than an RT.
As an RT you can work in clinical care. You can get your board certifications to specialize in NICU/Peds, Sleep, PFT, and Adult Critical Care.
If you're lucky you'll work at a place with strong protocols allowing you to manage your patients fairly autonomously (for example, physicians don't place RT orders at my hospital. They write for an RT eval, after which we do a full assessment and order whatever's needed. Inhaled bronchodilators and steroids, oxygen, physiotherapy, BiPAP/CPAP, ABG's, and CXR's). You might get to work at a place where the RT's get to do a lot of procedures such as ECMO, IABP, VAD's, CVC's, A-lines, Intubation. Still though, this is all just clinical work.
Advancement is usually limited to direct line management. Larger departments will often have an educator/coordinator - but usually just one. Large research facilities often have RT's who do research, but again, a very very small number.
You can become a Case Manager, Asthma Educator, Nicotine Counselor, Pulmonary Function technologist or Sleep Therapist...but again, these aren't advancement opportunities so much as lateral moves.
As an RN? You can do almost anything, almost anywhere. It's not worth trying to list everything a nurse can do and everywhere a nurse can work, because it's almost endless. You will have far more opportunity than the average RT will. And keep in mind - this is coming from an RT who loves his profession and job and wouldn't ever give it up for any other healthcare career.
So...which sounds more appealing to you?