I have driven over 9 a couple of times, but never towing. The route I HAVE towed is to take 24 to Leadville (no need to go to Vail) then Colorado 91. The pass at Climax is Fremont--no problem for me. Then a rather straight descent into Cooper Mountain and I-70. I've made this route many times towing a 34' fifth-wheel. I mention this because there is no need to go by Vail.
I'm not saying this is better than 9. It is just that I have never had occasion to go that way towing. It certainly does look more direct. At least you have some alternatives. Maybe someone with experience towing on 9 will chime in here.
EDIT to add. Poncha Pass is an easy pass coming down into Poncha Springs near Salida.