Monthly Price. These systems have monthly subscription fees.
Consultant recommendation. If you are working with a consultant, what does your consultant recommend? If you want to use a system that your consultant has not worked with yet, do you have the patience for your consultant to learn the system?
Look and feel. Are you okay with limited choices on customizing the look and feel of the portal? Do you have a developer with the ability to take advantage of advanced features like custom html or css?
Mobile experience. Do you need a system that works on mobile, desktop, or both?
Level of access control. Do you need edit access? Do different types of users need different levels of edit access? Do you need edit access by users who are not logged in?
Number of users. How many external users will you have? Most systems have different pricing levels for different number of users.
Number of developers. Will you need only one person with access to developing your system? Or will you need multiple users, such as you and a consultant?
Support & community. What level of support do you need?