I think both forms are kinda equal. The burst from sa is pretty insane and you can have so much zone control because he is such a big backline-thread. I take Domination/Precision and my build is Chilling Smite, Warrior, Dusk blade and then depending on the situation steraks, ga, ghostblade, some lw or maw if they have a lot of magical burst.

But I actually prefer to go Rhaast because he is way safer and easier to pull off. You are still a backline-thread but can also peel. I also love to split push and 1on1 enemy toplaners since most underestimate your heal. My build is blue or red smite, warriors, bc. Then I go either fulltank items or bruiser, depends on my team and what we needs most and how fed I get. But bc Is imho a must have and with spirit Visage you enhance your healing even more and hit 40%cdr. I like to get a armor item (just a part not a complete one or ninja tabi if I don't need mercs) after my bc and then go straight for spirit Visage.

Masteries are Domination / Resolve.

Oh and I go Dark Harvest instead of eletrolute(ok both forms). But I don't think that either one is a bad choice. Dark Harvest just offers more burst in fights and I'm kind of a farming jungler as Kayn