Samsung makes some good drives, I have the 850 Pro, but on an external drive it will be the interface that dictates speed - if that is what you are looking for. I doubt you'll realize any speed benefit going with SSD over traditional
If you own a Seagate GoFlex Thunderbolt portable adapter, it is possible to add your own choice of SSD. It will be very fast, however I think that with the portable adapter there can be instabilities if you use a drive larger than 512 MB.