I admit I don't know much about the breed or breed characteristics except from pictures I've seen posted here or online.
But based on a horse is a horse...
I like the frame, the shape and confirmation of the palomino better.
To me, it just flows together better.
I look at needing a wither to hold a saddle better during endurance rides.
I look for a bigger bum and motor hind end.
I look for a leg that will give power yet be ground covering in distance covered per stride.
I look for a deeper heart-girth so larger heart and definitely lung capacity possible.
I look for a larger muzzle and wider profile so breathing under stress is easier and more capacity travels the sinus passages.
The palomino to me just flows together easier and has more development than the other for both being yearlings.
I look for a thicker bone for what can be "pounding" work over many miles for less injury to occur doing endurance riding training and events.
And, I know at this age it can all change near overnight...
Enjoy your search...