I think alot of longevity of media depends on the manufacturer and the quality of the dye they use.
For Dual Layers, I haven't seen any DVD-DL, but I've used DVD+DL. Verbatim seems to be good - I've done maybe about 9 DL discs w/ verbatim and no problems. I started to have some issues recently but that's because my burner is dying.
This typically goes for all media, CD, DVD, like a poster said before. For CD's I don't find that its a big issue, I've done a number of brands w/o any problems. For DVD's, I find that Verbatim is good. I've used some Imations that didn't last too long and gave me some coasters. I have to say, Verbatim has been the best so far for DVDs.
As for finding DVD+DL's, newegg has some for sale. I've found some in Office Depots, Frys (ew), but primarily I've found that newegg has the best price for verbatim DL's.