I can't answer your question technically, but from what I understand practically speaking, the 29's will have more penetration, more intensity. So it depends on your growing style. If you grow tall plants, you might want the 29's. If you grow in a scrog, the spread out light of the 250/18's might serve you better. From comments others have made, it sounds like the 18's came out first, and then the 29's were a "next generation", bigger better brighter thing. I've also read that the 18's are more efficient... so I don't think there's anything particularly wrong with them.
I'm running two 120's right now -- which is 4 Vero 18's -- in a 2.5x2 space. First run so I can't give you harvest numbers, but its bright as hell in there... if that helps.
Sorry I can't be more specific, I've gotten into this late in the game and I'm still in the early part of the learning curve myself. Hope something in there helps.