Corsair is well known and reliable. OCZ has been known to have lots of problems for some time, but now is under new management and seems to be considered good by the people who've risked going back to them.
The "dual DDR" or "twinX" versions are not any different than the regular modules. They're just two modules that were plugged into a motherboard and tested together and qualified as "matched" memory. If it's only a few bucks difference, the twinx package is good just because there's a guarantee to work in dual mode, but otherwise just 2 regular modules will work fine.
Also consider Kingston HyperX modules. They're a bit cheaper and just as good.