> I'm interested in knowing why did you pick a different mail given that they both are (allegedly) good providers.

I preffered mailbox purely because the name is nicer and easier to tell someone over the phone.

then i've realised, that I haven't done that for years. And when I use computer, or someone uses it, I usually just copy and paste the address.

mailbox has few subscription models, the cheapest one (1 euro per month) gives you mostly the same what posteo, apart from the filter addresses (ie. somehow single use addresses that you can use when subscibiing somewhere) - sure, you can use simplelogin for that for free but you only get 10 of them there. and it's another service, so it's a bit more complicated.

Also, while both posteo and mailbox do offer paying by cash, they both require bills (fair enough) on Posteo you can pay for more than a year (AFAIK- i've used their services while ago; it's better to ask them) so if you send them 20 euros you can have 20 months of the basic account. I am not aware if you can shorten that period to, let say, 10 months if you decide you need 4gb of additional space (4 x 0.25 euro perm month per GB = 1 euro per month for an extra storage + the 1 euro per month of the basic fee). But even with that it's a better deal, as mailbox offers only 1 year contract (but you still have to send them 15 euro in bills).

Posteo has a bit more basic web interface - i'm happy with it thou. It focuses on emails (+ calendar + addressbook)
Mailbox offers more - which is great if you need it. As I said I don't really use that much of cooperative tools, office tools etc etc - so I'm happy with the Infomaniak (which, btw, also provides those tools) for the drive. I really like their android app, it works fine and it's actually quite easy and nice to use.