1. Playstyle -> 2. horse -> 3. build
The choice of horse should be made by how you want to play and what you want to be good at.
After that you can choose a build.
Lance cav can be very different depending on horse, build and equipment.
If you want to be good at fighting other melee cavalry you should definately go 18/21 or 21/21 and use an arabian warhorse, for example.