Dropbox is broadly compatible. iCloud is Apple-centric. Both can be easily used to put photos in the cloud. If you want to use the space for some other things, the much bigger world beyond the wall will be more likely to already use Dropbox than iCloud EXCEPT on iPhones in that world.
If you can refer some new customers to Dropbox, they will give you free added space. I have about 12GB of free Dropbox space. That's more than I ever need for anything I want to transfer or store in the cloud.
If me, I would NOT store photos in the cloud. They will hog a lot of space to mostly just sit there. Instead, I'd purchase a couple of big drives to get a couple of recent backups of all photos (and my Macs) on them with tools like TM, CCC or SuperDuper. Store one such drive offsite and regularly rotate it with the onsite one to keep the offsite "last resort" pretty up-to-date for a worst-case recovery scenario.
I MIGHT store latest photos temporarily in the cloud if I couldn't back them up to 2 places locally until I could get them on the offsite drive. But as soon as I have them in at least 2 places- including the offsite- I'd purge them from the cloud to free up that space.
If I really need photos in a cloud for some reason, I'd spend some money on a Synology NAS or similar, use its "my own cloud" setup wizards to then own and run my own personal cloud... and store my photos in MY cloud there. That would cost more than short-term rent for Dropbox or iCloud space... UNTIL the rents eventually exceed what it costs. I would NOT do this solely for photos but to take advantage of many benefits of a Synology NAS which includes photos and personal cloud, PLUS many other useful capabilities.
Personally, my view is to use any Cloud storage as "extra" vs. any essential part. HDDs of size are dirt cheap. It doesn't take much more to own a little NAS (and thus your own cloud). A good old portable drive/stick could be another way to carry a physical "cloud" in your pocket (which will offer much faster read/write than cloud options).