That's like comparing rsync to google drive. One is an open source tool where you can use whatever back-end you want, the other is a service. (Which is fine, just different kinds of things.)
However, in this case it's the open source tool that has a much easier user interface (I am actually proficient with tar, but still my tarsnap experience is like comparing 'restic backup /my/files --repo /mnt/backupdisk' with https://xkcd.com/1168/)
Indeed. Restic is just something you apt install and nobody provides you any service (you have to organise your storage space yourself); tarsnap is not simply free to use for yourself with your own storage. (Not saying it has to be free, but that's what makes it the definition of a service you have to purchase.)
However, in this case it's the open source tool that has a much easier user interface (I am actually proficient with tar, but still my tarsnap experience is like comparing 'restic backup /my/files --repo /mnt/backupdisk' with https://xkcd.com/1168/)