If you get the chance to go fixed wing then you should absolutely go for it. I did OH-58D’s for 9 years before making the jump over to fixed wing. Since then I have flown the RC-12 (King Air), EO-5 (Dash 7), and UC-35 (Cessna Citation). It is a move that I have not regretted a single day since I made it.
One of the other posters mentioned that the fixed wing community can be very senior, and I can attest to that. I just came from Wiesbaden Germany and at one point we had 6 CW5’s in the company. You heard that right, in the COMPANY. With that many senior guys in the company you end up with CW4’s doing jobs like supply and if you’re really lucky, safety or perhaps flight ops.
With regard to promotion rates, nobody knows what the future holds. I can tell you right now at least the fixed wing community is VERY fat on CW5’s. Because of that, last year’s board placed a ceiling of 4 on the number of people they would promote to CW5. Given the number of people that were up for the promotion board, that was an 8.5% selection rate.
Nobody knows what Army Aviation may look like in the next 5-10 years. Just make the best decisions you can for you and your family. Hopefully it will all work out for you. PM if you need more info.