+1 HD is bigger bottleneck. SSD means faster, more battery, less heat; all things better for laptop. Will help a lot with large file reads like Final Cut.
I agree that a SSD will likely give you more bang for your buck than a RAM upgrade based upon your use. I don't believe the video editing is as memory intensive as photo editing, although I could be wrong.
You might take Buddyme2
's advice of looking at the Activity Monitor Panel or iStat Menus (you mention you use it in another thread) to see what happens to your memory usage during your typical activities before you make your final decision. While I suspect you likely will be fine in terms of memory as long as you are not running a lot of programs and/or have a crap ton of browser windows open, you might be pushing it when you do video editing.
FWIW, I am only using about 1.7 GB of my 16 GB of RAM at this moment. That is with about 20 or so web windows/tabs open as well as iTunes (with music playing), Apple Mail, BusyCal, and Chroma actively running plus a handful of memory resident programs (iStat Menus, Bartender, Dropbox, Box, VirusBarrier, Netbarrier, and a couple others that I am likely forgetting).
Plus, with the boot drive as a SSD, even if you run out of physical RAM, it will resort to virtual RAM (i.e. using your hard drive...or if you upgrade your SSD, then the SSD). And virtual RAM will be faster with a SSD.
For storage, I would suggest external HDD. My question would be USB 3.0 vs Thunderbolt. Since most external HD will be HDD, I don't think USB 3.0 would bottleneck SSD > HDD and give you more options to use it on non Mac device.
For a external hard drive for storage, either would be fine. USB 3.0 will be cheaper than Thunderbolt.
If going for an external SSD, USB 3.0 in theory should not bottle neck most SSDs, but theory only goes so far. For example, the OWC SSDs I recommended/link to have max read "speeds" of up to about 560 MBps and write "speeds" of up to about 525 MBps. With USB having a theoretically maximum throughput (that means in reality lower) of 640 MBps, USB 3.0 is only the bottleneck if the actual USB throughput is greater than about 88% of the theoretical maximum. One of the OWC's 2.5 drive enclosures sold with a 2.5" SSD (which would be one of their SSDs) inside of is listed as having USB 3.0 "speeds" up to 500 MBps. So, it looks like USB 3.0 might slightly bottleneck a SSD.
Of course, buying a large capacity SSD (whether for internal or external use) is a rather expensive proposition.