There are a number of threads that touch on choosing between these applications. I thought that I'd set out the reasons my organisation recently chose FastMail, because our reasons don't seem to be reflected elsewhere on the forum.
Historically, I've used Microsoft for work (365 Enterprise, fully loaded), and Apple for personal projects (Mail, Contacts, Calendar, Pages, Numbers & iCloud). For personal e-mail, I've used, in addition to Apple (john.doe@icloud.com), both Google (john.doe@gmail.com) and Microsoft (john.doe@outlook.com). On the cloud storage side, I'm pretty familiar with DropBox in addition to iCloud.
I'm currently involved in a start-up that needs a business web site (ultimatewidget.com) and business e-mail (john.doe@ultimatewidget.com).
We have decided to go with Apple hardware (computers, phones, tablets) and productivity software (Pages, Numbers, Keynote & iCloud). We would also go with Apple Mail, but it does not support private e-mail addresses such as
john.doe@ultimatewidget.com. Nor do Gmail and Outlook.
We narrowed our options to FastMail, G Suite and Office 365, and we gave each a trial run. These all cost pretty much the same per user. The decision came down to features, but with an outcome that is perhaps not obvious.
Having chosen to go with Apple productivity tools and cloud storage, the problem with G Suite and Office 365 is that they include tools and storage that are not only redundant to our needs, but being built-in are quite liable to create confusion. Sometimes choice gets in the way of simplicity, clarity and focus. In the case of G Suite's Gmail, we also decided that we preferred e-mail folders to Gmail's labelling system.
In the end, we decided to go with FastMail. It does one thing and appears to do it well. Most of us also intend to use the FastMail app on mobile devices in order to help separate our business e-mail from personal e-mail. It might be worth noting that this desire for a dedicated app was one of the reasons that currently trendy ProtonMail did not make our shortlist.
The outstanding question is whether to use Apple's or FastMail's Contact and Calendar functions. I haven't quite figured out the implications either way. Given that we'll be using iCloud for storage and sharing, I also haven't decided whether there is a role for FastMail's Files storage function. My gut sense, perhaps wrong, is that it is a bit anachronistic in an age of ubiquitous cloud storage. It's unclear to me what the practical use case is for it.
It's also important to point out that we intend to use e-mail mainly for communication with people outside our organisation and perhaps for particularly formal discussion within. Inside, we'll mostly use instant messaging, possibly Slack but more likely Workplace by Facebook. This reflects the diminishing role of e-mail, and the increasing role of messaging, within organisations. As part of identifying a messaging app, we also looked at TopicBox, a product related to FastMail, but in my view it is too close to traditional e-mail - maybe good for a group e-mail list, but not for fast, informal, brief communication. The great advantage of Facebook's Workplace over Slack - and I say this as someone who does not have a Facebook account - is that just about everyone knows how Facebook works, which means that it requires no training. For non-tech people, which includes an important part of our staff, Slack is just too tech nerd oriented.
Basically, we selected an e-mail provider by looking at our written communication needs as a whole rather than at e-mail in isolation. Perhaps paradoxically, that resulted in choosing a specialised e-mail provider. In any event, two weeks in, I'm very happy with FastMail. Compared to dealing with G Suite's Administration Consol and Gmail Settings, it is a pleasure to work with FastMail's back end, even if there's a bit of a learning curve.