Go for the Sony. It'll likely contain SONY16D1 or SONY D21, which are good MID's in my experience. If you're lucky, you might get Taiyo Yuden discs.
Never buy Memorex. They usually contain Ritek or CMC Mag MID's, which are awful in their own regards. The former has poor longevity while the latter has terrible burn quality. You could also get Ricoh, which is decent, or Moser Baer, which can be good if it weren't for the poor quality control. On the off chance you get MCC, they'd be the best discs, but they're rare with Memorex.