Imo I would always go for an internal drive over an external one if you have space and that's true even if you have have thunderbolt port. If you are short on money check out ebay - people often pull the smaller ones that come with the computer and put in a larger one.
And, again, the advantage of the smaller ones is that you can add to it later. Setting up a RAID is a daunting task (backup everything and have a tech savvy friend help!) but once you do the difference between single HDD and SDD RAID is night and day. I no longer think about HDD loading times - I only worries while playing are "how much heat can my GPU tolerate"?
