1 old (Snow Leopard) iMac & 2 newer (Mountain Lion) MacBooks -- iCloud or DropBox?
I think in your case Dropbox is the solution to keep synchronized your files.

You can continue to use iCloud on your other Mac with Moutain Lion (to synchronize your reminders or other contents that use iCloud).

Moreover, my understanding is that iCloud don't sync your Documents folder, so anyway for your need, you should use Dropbox.

However, be careful when you put your personal data online !