Synthetic benchmarks aren't everything. Game performance is different for every rendering engine. Unreal 3 Engine and Cryengine are examples. Take this into account when you decide.
http://www.techpowerup.com/reviews/MSI/HD_4850/17.html
http://www.techpowerup.com/reviews/MSI/HD_4850/10.html
4850 performed better on most other game engines though.
As for "manufacturers," they're actually just distributers at this point. The cards are made on the same factory floors, and sent to distributers such as Sapphire, Diamond, and Asus, where they past on different stickers. You have to wait a few months for non-reference cards to come out.