I guess it all depends on what you are looking for in a foal. If size is a consideration the Traveler should be considered as it seems they are larger in body and height. I have 4 Rocky Mountain horses. They can be on the small side 16 hands being the top of the registry rules. I don't know much about the Travelers except what you probably already know from the web. They just seem larger than the Rocky's I may be wrong.
The Rockys are easy to train, very people oriented, surefooted and couragous. I trail ride and have gone many miles on my girl without hesitation. Her baby is calm, takes everything in stride and has been very easy to train so far. If color is important to you and you are wanting the desired Rocky chocolate flaxen you have a good color in your mare to get it. I will post a picture of our little family. The stallion on both of our foals is a dark bronze body and flaxen mane/tail so had the silver gene.
Both of our girls have good "cow sense" we have used them a couple of times with the neighbors cattle.
What traits are you looking for in a foal? That might help in answering your question a bit better.
Vida is on the left and her baby is the choc/flaxen she is similar in color to your mare. Sassafras is the sorrel and the monster is her baby. We are afraid the tall one will go over the 16h when she is mature.
My mare is a Quarter and she's out of reining stock, but she has the best feet of any horse I've ever seen...like barefoot on rocks all day with no soreness or stiffness! It's like she has mustang feet, and she's a smokey black grulla.(technical!) so I think she'd throw a really pretty dark foal if the sire is dark, or chocolate flaxen like the rockies.
I want a horse that's calm and gentle, beautiful in the hills with no shyness (I know that comes with experience too) I would want the foal to have sense, not like a dumb TB, no offense to anyone!
I had also thought about breeding her to a Mustang, but I'd want it to be taller, my mare is almost 15.1, so anything taller than her is peachy.
I honestly don't think you can go wrong with either breed for what you are wanting in a foal. The Montana Traveler might be easier to find a stallion in your part of the country and I think you could even register a foal with them though I would check to make sure. I don't t think you can register a Rocky unless both mare and stallion are certified to breed not just registered with them. Good luck in your search for a stallion and the whole breeding process. Theres nothing like having a foal. :)