1. They are all roughly the same age;
2. Papers/breed/bloodlines matter less in Canada right now when compared to the US horse market;
3. Mares have a slightly larger market because of their ability to produce offspring.
My first thought would be to put 30 days of training into the horse that currently has the least amount of training. To me, this would provide a larger increase in "value" from a sales standpoint. Smallest investment for greatest return. So unless one of the mares will improve a bunch, I would say send Ben.
Emmy is pure bred Arab, and nicely bred, well out of a super mare anyway
Willow is Sadllebred x Arab, registered as half arab
Ben is Perch/tb x Saddlebred
The girls have papers, and are broken, and with a little work on them will both increase in value.
All Ben has going for him is a great color, nice size and a beautiful face, so you are correct, if he has a solid foundation put in him it will protect him and give him a value, because when it comes to it a grade gelding can only rely on what he can do to get him through!