Your priority should be on the SSD, that will give you the best performance boost for the dollar. A 250GB Samsung EVO costs less than $100, you can get a 500 GB Samsung EVO for around $160. If you are close to using your current hard drive space it might be worth it to get the 500 GB SSD and save up to upgrade the RAM later if you are not able to do both at the same time.
But for about $200 you can do the 500 GB SSD and the 8GB of RAM. OWC is more expensive because they are more Mac specific, but Crucial will work perfectly as well. I would have no issues buying the Crucial upgrade.