I've used both Kerasilk and Dry and Porous.....when I said the Definition line I meant the Dry and Porous conditioner, specifically. Sorry I didn't say that. That said, I loved both.
If I were deciding between Kerasilk and Dry/Porous, I would base it on how my hair felt right now. If your hair's really dry and in bad shape, go for the Dry/Porous....if it's in rather OK shape, but you want it at it's best, go for the Kerasilk, IMHO. I switch between both as I need. Either formula will last along time 'cuz you don't need to apply as much (it seems to me, that is) as w/ other brands of conditioner for the stated effects.