I'd keep Sauron, he's bigger and would look more impressive in a PF collection, he's a lot more detailed than the ML and no matter which character you prefer Sauron is more iconic, especially with him showing of the One Ring.
But if it's that though a call for you, why not keep both? I'm not familiar with ebay prices nowadays but I think it's unlikely either of them will get you a price that justifies having to make this hard decision. Why don't you display the ML outside your collection, e.g. on a desk? He looks awesome all by himself.