I'm currently considering a switch from ext4 to ZFS and am having trouble finding information about whether or not it uses more or less space to store the same amount of data as ext4. My understanding is that it uses checksums for data integrity, which I'm assuming comes at a cost of more physical space used when compared to ext4. Since it seems nobody is talking about this, I wonder if perhaps the compression (which I understand is enabled by default) more than makes up for the space used by the checksums?
The checksumming and space used by it isn't a real consideration. That's why nobody is discussing it. Why are you under the impression that it would take up more than a negligible amount of space? Did you read something to lead you to believe otherwise?
This is easy enough for you to test on your own with your data, though.
I will say that compression (use lz4, please) is worth using on just about any ZFS dataset.