Buy a Lord Kroak (proxy as a Regular Slann) and can swap and off the palanquin with a priest (from the Trog kit). Have all 3!
Sooner or later you must have both of them, so start with the one you like more. From a gaming pov, they are both good and bring useful tools at the table. Personally, I've a slight preference for the Starseer, but it also depends from a lot of factors (for example, if you plan do field battalions that can be better buffed by a Slann)
I'd start with starseer. He will have a place in 5x the games the star master will. I think it depends a lot of what size games you play. I would t field a starmaster below 2k points personally.
@Mark Sinosich's answer is the big winner but Starseer is definitely part of way more lists than Slann. To be precise, as it is right now, the only way to have a Slann in a list is to build it around him while the Starseer can have many uses