When it comes to external storage, the best that you can do is to get a server close to your Chevereto server. This is because the system will connect from your Chevereto server to the remote SFTP server.
If your server is in USA then get external storage in USA. Nowadays all servers have very decent network speed, but it will be day and night if your server is in USA and your storage server is in Europe or Asia.
So first things first, get something close to you and then evaluate based on price and performance. Some servers could be next to you but with an awful network, hardware, etc.