If all your devices and computers are Apple, then the iCloud syncing is a good option. It's marginally more secure than DropBox due to the files being stored on the less visible to the public iCloud servers.
If you use devices and computers from multiple platforms, and/or would like to be able to access your 1Password database via a web browser, the DropBox options is better, more flexible. The difference in security is trivial, since the file and transfer methods use some of the most advanced encryption available.
Put it this way. With iCloud it is slightly more difficult to get ahold of the file that you would need a supper computer and dozens of years to decrypt.
Be sure to pick a good master password. This is your weakest link. If someone gets that it doesn't matter much which service you use.
And, you can always change your mind latter. It's not terribly difficult to switch the syncing methods; though you have to do it on each and every device.