I've chosen Ioirveth for my first playthrough, partly cuz I helped elfs in the first Witcher every single time I replayed it (I did that 7 times lol, never joined the Order), now I'm playing it the second time and I've gone with Roche.
To be frank, joining with Roche seems the most reasonable thing to do story-wise, I mean, I liked Iorveth a lot too, he's an interesting guy and some moments in Vergen added some nice touches to his persona (like viewing his dream, where he sees himself as a wealthy guy eating from an oversaturated table xD), but joining up with him just seemed like a really odd thing to do. This guy and his men has been nothing but a bother to Geralt durning most of the hours of gameplay leading to the choice momemnt.
But I've seen far more ridiculous choises in games (for one I never undestood the motivation to join the Order in TW1, for me, joining up with them was a much more odd decision then joining Iorveth in TW2), so I don't really have a problem with that, besides, part of the reason to join Iorveth is Zoltan and his desire to aid his bretheren in Aedirn, especially when he meets his other dwarf buddies, that makes up for the lack of reasoning in the begining a lot (it actually becomes much more reasonable then helping Henselt lol).
If I will play TW2 a 3rd time, I will definitely go with Iorveth. Helping out Saskia and Zoltan just seems like the right thing to do (not to mention Saskia is really awesome and discovering the little secret that she had was really surprising).