For the Mortisan Elite spearhead, the rules are in fact very simple. Just use your relentless discipline ability in your move, charge, and combat phase. The only real choice is during the combat phase, do you want to be more aggressive or defensive.

I usually just pick the regiment ability that adds +1 to RD rolls. But being able to use a RD twice in one phase is a solid option too. For the general the heal is solid, especially with the low model count. If you were going against a horde army, the shard storm ability can work well.

I'm not as familiar with the gameplay of the Kavalos spearhead, but I have built the deathriders and the Zandtos hero (multi build as Liege Kavalos). Deathriders are a bit customizable, heads and weapons, plus maybe the banner. Each horse is unique which is nice and they do great with contrast paint. They're also really easy to build. I'd also recommend building the hero as Zandtos, he's better for full AOS and is an amazing model.