Is it the new 800 MHz FB-DIMM, or the older MP 600 MHZ? If this is for a MP (early '08) you must use the 800 MHz. To answer your own question, did you check OWC's prices?
The general consensus is, go with OWC for RAM upgrades, but hold on to your original Apple RAM in case you need to have your MP serviced or for warranty work.