4gb of RAM from OWC only runs $33 for the Late 2008 model 5,1 Aluminum Unibody and the Late 2009 model 6,1 and Mid 2010 model 7,1 White Unibody and it's Mac tested. http://eshop.macsales.com/shop/memory/Apple_MacBook_MacBook_Pro/Upgrade/DDR3_Whi te
There's no particular difference in the RAM for a given specification. However, Macs can be quite finnicky about the quality of the RAM.
Both the suppliers mentioned above are good, and not expensive.
Do not be tempted to use cheap RAM from eBay or Amazon MP, many of these are factory seconds.
If buying Kingston, use the premier range only, not the ValueRAM range.
Also, the slots on Macs seem to be quite a bit tighter than most PCs and it can take some force to seat the RAM correctly - make sure you feel the soft click as the contacts fully engage.