I'll throw out the following as a starting place for discussion.
I think there are three major components to this question that you need to understand and answer before you get into finer points like:
What's management's view of the Cloud? -
All for it? totally against it? Your CIO might already have your answer for you.
What's your network speeds like, your ISP speeds? if your ISP is still on dial up then AGOL is DOA.
Company growth rate? etc...
I'm sure there are a lot of other things to be considered.
But the three major things that were involved in our decision to go with Portal were:
1. What is the status of your data?
a. All data is for public consumption
b. All data is confidential and must be protected
c. Combination of confidential and public data.
2. What is your staffing level?
a. Do you have sufficient staff and are they technical enough to support standing up Portal and keeping it running from the IT side? Building up servers, installing software, updates, etc...
b. Ditto for the work load of supporting the AGS Feature Service, Map services, etc... This is probably about the same across Portal and AGOL but I suspect Portal might require more in house resources (time.)
3. What's your infrastructure level?
a. Do you have sufficient hardware resources (hard or virtual) to stand up Portal, a Data Store, Integrated Server?
(this of course is dependent on size of your organization, etc...)
b. Do you have sufficient financial resources to handle all of your named users? This one is irregardless of Portal or AGOL. Both require named users and the cost is the same. But one AGOL license does not also mean one Portal license. They are independent. If you already have AGOL licenses, I believe you can transfer them to Portal if you go that route but that's between you and Esri.
I think if you answer True to 1.a, then stay with AGOL.
Why replicate AGOL in house if you don't have a burning need for keeping data and processes behind the firewall?
AGOL is kept up to date for you and new advances show up there before they do on Portal.
Let Esri be your IT staff if you can.
AGOL will scale as you need more resources. Portal you're responsible for the scaling issues.
If 1.b is True, then Portal is almost a given.
Yes, AGOL has security built in and you might be fine using restricted access but most organizations in this situation deal with security by keeping it in house.
And if you're in 1.c then you have to consider 2 & 3.
Do you have the resources to build up and support your own Portal?
It's not a trivial thing to stand up. It's not impossible by any stretch but when you have AGOL sitting there for you, pretty much ready to go, then I think you need to have a compelling case for adding to the company work load and resource drain.
There's my quick two cents worth.
By using portal you would not have to worry about setting other security on services that are meant only for internal use by using portal for your internal users, where you would have to secure these services in AGOL.