The benefits to buying a SSD as an option from Apple are generally that (1) it's already installed, (2) it's covered by your Apple warranty and AppleCare if you buy it, so if you have problems, you can just go to Apple, and (3) you can be confident that the drive works well with your MBP.
The drawbacks are generally that (1) you pay a significant premium -- usually more that double -- compared to similar (or even the exact same) part bought aftermarket, (2) you have to install yourself, and (3) if you have a problem (bad RAM stick, early HD failure) you have to go to that manufacturer rather than Apple.
Installing a new drive isn't that difficult -- Apple considers it a user-serviceable part and they have directions on their site. Better yet, ifixit.com has lots of guides on how to do all these things as well as selling the less common tools you may need.
Regarding SSDs, however, there have been some concerns voiced that the types that Apple sells (made by Samsung) are more susceptible to performance drop-off over time than other SSDs that use a SandForce controller, like OCZ's models, including the Vertex line.
Take a look at the ifixit site and consider saving some $$.
Also, that Vertex 3 was just reviewed at anandtech.com and is expected to be an absolute monster of a drive -- nothing that an Apple-bought SATA II drive can even hope to match.