If you really had to sell... go for the youngster first. As long as it leads reasonably well and you can pick it’s feet up for a trim you shouldn’t have any issues selling.
Next to to would be the mare. Ideally wait until spring and the weather is nicer/hopefully no covid lockdown... get her...
Speaking having been in a similar situation. Do you want to keep riding? Assume answer is yes. Why do you want to keep riding, just to enjoy being in the school and with your horse, or because you want/need to be going to competitions?
if it’s the second answer, I’d suggest you feel that way as...