Apple's SSD prices don't look that overpriced this time. But of course if you can find a better SSD with a similar price go for aftermarket. OCZ Vertex 3 is coming.
Given that the upgrade to a 128 GB SSD only costs $100 when ordering from Apple, I'm not sure where you think you can find a cheaper option I sure can't and I'm somewhat reconsidering my choice to not get it.
The 256 GB upgrade seems basically on-par with 3rd party offerings - so that's a toss-up. Get it from Apple and get the warranty or get the std. drive and get an SSD from a third party and have the original drive as an extra/external.
Personally, I'm considering re-ordering and getting the 128 GB SSD and then swapping out the optical drive for a std. drive. That'd allow me to keep the OS and apps on the SSD + swap space and then all my user files on the 2nd drive. Doing that is about $100 cheaper going from Apple rather than a complete aftermarket option.