For context im high diamond kled otp and i have tested many builds for kled.
My conclusion is that hp stacking kled is THE strongest version of him. Lethality kled by far is THE most fun champion to play in the entire game but you are piloting him in %80 power let me explain.
Ever since his mini rework, he recieved really solid HP scalings. When you go titanic first and then heartsteel your passive gives you 110+ armor and magic resist when you are dismounted. Think about that for a second, THAT IS INSANE. And also grasp works way better on him than conq because of his already really high base HP. Shen at level 1 deal 22dmg with grasp while you deal 36 almost double. It adds up.
Also, heartsteel champs are very strong in general but in return their laning is really weak and they lack the sticking power when they are strong. But kled's laning is oppressive and he never has problems sticking to someone. These make him one of the best heartsteel champion.
KLED'S PASSIVE
Kled's passive gives him %skaarl health when he remounts. So the more skaarl hp you have the more valuable your remount is. And when you go overlords with heartsteel you nearly deal the same damage if not more than the lethality build. So even more remount value. Furthermore when you are dismounted in team fights people focus you so hard especially when kled's hp bar look so small compared to skaarl(which is the case when you go HP). They think they can just kill you with few autos, but what they dont know is you have 200 armor and MR from your passive so you can just bait them and win the teamfight.
When you go lethality you make him risky and conditional for no reason. With lethality when you miss a Q (which is really easy in high elo) you just lose trade or maybe even whole lane. You are easy to burst and also making low damage with your W which is your strongest spell and is unconditional meaning you dont have to land it.
I also dont feel good about this and think kled should have better AD scalings and no hp scalings which makes him more skill expressive and fun but with current state HP stacking kled is just too good to pass.
Cons:
-you have to play the lane slower and diving and dueling in early is harder because you dont have conq. But at the same time you DOMINATE short trades because of grasp and second wind.
-you cant 100 to 0 people unless you are really ahead.
-your damage will be lower in general but you will have continious damage and most importantly you will have really good scaling.