Sollir:
It was kinda late last night. While I didn't mention the +2 Dex/-2 Con in my drow/doppleganger post, I did factor it in my next post.
I wouldn't say the half-fiend or the drow have better skills. The half-fiend has more skill points, but the same level as the doppleganger. The drow clearly has 2 more HD, but less Int, and of course, weaker stats (lower skill bonuses). Of course the doppleganger gains like an additional +14 disguise as a racial ability, not to mention a +8 bluff (if his detect thoughts is active). While these bonuses don't exceed the half-fiend's, they are very very useful in some situations (albeit, rarely in combat).
Furthermore both the half-fiend and the doppleganger probably qualify for Assasin next level. While the half-fiend has more skills, the doppleganger has a slightly higher BAB (thanks to his monstrous humanoid levels). The doppleganger also has more hp and far better will saves. The half-fiend is truly abysmal at will saves, and its something like an achilles heel. For the drow its still bad, but at least some racial abilities help to overcome it.
Offensively, the increased Sneak Attack (albet the half-fiend only has 1 more die) does make them slightly better in combat. However, as others have pointed out, it seems like the doppleganger, while having less sneak die, would tend to get more important/impressive sneak attacks off.
I may do a Doppleganger Rog2/Ass10 vs Drow Rog8/Ass10 vs Half-Fiend Rog 6/Ass10 later, but I have a feeling things will be much the same in terms of analysis. Less Sneak attack, higher hp, slightly better BAB, still has overwhelming spell-like abilities.
I did jack up the dop's feats too, I'll edit it.
Technik