Simon's advice in post 2 above is spot-on.
If you're unsure about how the upgrade will perform, MAKE SURE that you do one of the following:
1. Create a bootable backup using CarbonCopyCloner (or SuperDuper) first.
2. Update the internal drive.
3. If things don't go as planned, boot from the external drive and "RE-clone" it back to the internal.
By re-cloning you can "get back where you once belonged".
or do this:
1. Create a bootable backup using CarbonCopyCloner (or SuperDuper) first.
2. Run the El Capitan installer "aimed at" the EXTERNAL drive.
3. Boot and do some "test runs" from the external drive. Do you like what you see? Be aware that the OS may run a little slower (it's USB2), but you'll still be able to see if the applications you use work, don't crash, etc.
4. If you like it, then run the installer on the internal drive. OR, re-initialize the internal drive, do a completely clean install, and then "migrate over" your apps, accounts and data from the external drive.
5. If for some reason you don't like El Capitan, you could just erase the EXTERNAL drive, leaving your internal drive untouched.
FINAL THOUGHTS:
I would suggest you install a "straight SSD" instead of a hybrid for your internal boot drive.
El Capitan will run much better that way.
You'll just have to find this out for yourself...