I've run both (or all three, given I've run the White too). Usual comments with barbs follow.
Some points:
1. I'd take the Beige version over the White version at current points any day. It is a combination of factors but the key ones are the synergy between the Beige's rules package and the fact he can be the army commander and not an ally.
2. Two Wraiths are of course very good because they range from undercosted to undercosted and grossly over-powered (seen 'army advice' lately?). If you are running an Isengard list based on melee then they are a huge help just through 'Wings of Terror' (shame on all of you still using this spell
) and Pall of Night. Strength from Corruption is a nice Ace in the Hole too.I'd imagine that if you were running them as mastery 3 then the option of potentially getting all three each turn is pretty nice.
3. The Beige (Isengard) Saruman can easily make back his points as a mobile (Epic are mobile, place them well) artillery piece. Added to that, his rules make him a very strong army leader. He also can give hard-hitting melee units like Ferals or Orcs w 2HW a boost before they run headlong to make a hole in the line somewhere. When you consider that he can add rerolls, jump out, shatter shields on a target then hopefully weaken it further with a spell, then the hard-hitters crash in, he allows the Isengard player to really smash around an enemy. If your army relies a lot on interlocking specialist units, this can be pretty problematic to deal with.
I don't think it hurts much that Saruman makes more sense as an Isengard leader than two Wraiths either. In fact, since we are really just talking about stats in a book rather than any complex tactics, you have no doubt already realised that some of what Saruman can do could be acheived via other heroes.
So, inconclusion, I think the real question is not "Saruman or Two Wraiths" but instead "why not take Druzhag, a Wraith (being coy, I'll not say Khamul outright) and Thydan as army leader?" Afterall, that does not cost too much more than two Wraiths