How much data are you backing up? The compression and deduplication seem pretty good in Arq. My monthly AWS (and now Google Nearline) bill is usually around $2, so I hit the breakeven point vs the $5/month services pretty quickly.
My current backup is around 300 Gigs. I see the big advantage of Arq (over Backblaze) is the fact that you can choose your host. However their own hosting solution for $10/mo with a 250GB data cap is quite expensive. I can see that for smaller amounts of data Arq is more interesting, especially since it also stores history (which Backblaze does not).
It's worth noting that Arq can back up to Amazon Cloud Drive, which offers unlimited storage for $59.99 a year. As soon as they offered that I switched from Glacier storage; Cloud Driver is faster, cheaper and less fiddly.
That is quite good! Unfortunately, in France there is no Unlimited Everything plan. The prices are similar to the UK (comment from mobiuscog).
However there is one last quirk I have with Arq + Hosting Service. If you need to retrieve your backup _fast_ you will have to reach for a paid service such as AWS Snowball which seems quite expensive. I can't to seem to find more recent information about them shipping smaller disks, just some old articles.
I didn't even know they offered that until now. I viewed the prime advantage of Arq was selecting your host. And really that is "hosts", since you can have multiple backup sets and different hosts. Stuff that I often want to retrieve I might have on Google Drive as well as one of the "cold storage" options.
This being put aside I find their pricing a bit excessive in comparison to Backblaze for example.