First, I'd keep a copy natively on your computer (could be a USB flash drive, external drive, something, anything) and then a secondary backup on an external platter drive as your secondary backup. Do not -- not -- rely solely on a cloud backup. You'll regret it when something goes wrong.
Assuming you have a proper local backup, there is a terminal command you can use, and it is much better than using finder for big data moves (I'd rather not go into the reasons why, but permission screwups is one of several reasons). The command is ditto. So in your terminal command line, it looks like ditto [source] [target].
Also useful: you actually can drag and drop your source folder and target folder into the Terminal command line after typing ditto. That is you type ditto, then a space, then drag your Dropbox folder from the Finder into the Terminal command line, then drag your iCloud folder into the Terminal command line, and you're done.
Anyone with unix/linux/command line experience knows of what I speak.
Finally, if it was me, I'd keep Dropbox. Works great across Mac and PC, including iPads, iPhones, etc. Better than iCloud in my opinion.