My only experience with them was ordering a kit some years ago to replace my optical drive with a hard drive for my pre-unibody 3,1 17" MBP. There was a kit for the computer and an external enclosure for the optical drive. The kit for my Mac has worked like a champ--I now have twin Scorpio Black WD drives in it, and I can easily boot from either drive. It's harder to evaluate the optical drive enclosure because the optical drive was having problems. The enclosure actually seems fine too, since the optical drive worked just as well in the enclosure as it did in the Mac. At the time, OWC didn't have a kit for my Mac, but MCE did. But I have never ordered or had any experience with a drive from them. They have been around since 1993, and according to their company profile, they have been involved with Apple products since the beginning.
It might be instructive to call both OWC and MCE and ask each how their products compare with the competition.
Good luck! Whichever you get, you'll enjoy the greater capacity.