I wouldn't use any DL media for long term storage. If I was forced to, I'd use Verbatim.
Memorex's 2.4x DL media was utter garbage. I don't think I ever burned a disc that didn't have readability issues. Their 8x DL discs are a lot better though. I use em for the occational movie or game, but I still wouldn't trust them for long term storage.
"Blu-ray is just a bag of hurt." - Steve Jobs