Which FileVault 2 recovery option is more secure - storing the recovery key in iCloud, or saving it somewhere yourself?
Saving it somewhere yourself is more secure as long as that somewhere else is memorized in your own brain. Otherwise the answer to this isn't static.

iCloud can have vulnerabilities and so can Dropbox. Knowing about some vulnerabilities only informs you about those that have become public. Right now we might possibly know about a vulnerability in iCloud, but tomorrow that could be patched and 12 vulnerabilities in Dropbox could be revealed. 

I would consider them to be basically equal if you aren't going to memorize the recovery key.

As an aside, I don't memorize my own keys, I keep them in Dropbox using 1Password