If you want to spend the money on an external SSD then may as well just get the internal one instead.
I use an SSD, in an external case, connected via USB 3 as my OS drive, and have done so since I first purchased my machine (a 21' iMac). With Apple's SSD options being a RIDICULOUS price, an external SSD was a good compromise.
It works extremely well, and apart from the occasional glitch, and being a bit slower, it is identical to using an internal SSD. The internal drive in my machine is used as a data drive only.
If you do go external, just make sure to pick the right SSD (one that doesn't need TRIM activated), and that your external case is compatible for booting on OS X. You also need to be aware that you cannot install Bootcamp on an external drive, so if you are using Windows on your Mac, it will be on the slower internal hard drive. Updates for Windows on Bootcamp is the main area where you will encounter glitches, with the Mac booting into Windows rather than Mac OS X when you restart, until you hold down option and manually select, but this is a minor inconvenience only.
The hardware I'm using for this consists of a Vantec Aluminium Enclosure and a Transcend 256gb SSD (links below) both of these can be found on Amazon. The total cost of this combination is $85, while keeping the 1tb drive that is built in. By contrast the cost of an internal SSD option from Apple is $200 and you lose the 1tb drive.
To me this is a no-brainer. Given what Apple is charging and the reducing costs of SSD drives, having one externally makes a lot of sense, and it makes for a very easy upgrade later on, should you decide you want a larger or faster drive.
http://www.amazon.com/Vantec-2-5-Inch-Aluminum-Enclosure-NST-266S3-BK/dp/B00IAUPDFE/ref=pd_sim_147_6?ie=UTF8&dpID=41c+szlDHVL&dpSrc=sims&preST=_AC_UL160_SR160,160_&refRID=1S92T2BCSE3A0VDEBGZA
http://www.amazon.com/Transcend-SSD...81379&sr=1-3-spell&keywords=transcend+sad+340