If your prospective organization can't decide on what meetings platform to use, that's a red flag. Ask them what they use for communications. If they tell you Teams AND webex then you should consider ending the interview right there. This is an indication that they are a siloed organization, their IT department is lacking, and they spend without discretion.
It's an inefficiency for sure, but is also likely the sign of other things to come including a dwindling IT budget, unclear strategy amongst management, and indecision on their part.
Which platform is the best?
This will depend largely on the type of organization and how it operates. If there are a lot of files being shared back and forth, training sessions, and projects,Teams may be the best solution. If meetings are spread out and not as frequent with the organization instead communicating through email and chat then perhaps Slack is a viable solution. If communication takes place with external customers then Zoom might be your best option. Finally, if you have a global workforce where not every employee or meeting attendee has the same level of connectivity, the. WebEx and its dial in numbers may be a solution.
No matter what an organization selects, the point is that a decision should be made and preferable prior to you needing to ask them. The last thing you should hope to deal with when deciding on an organization is their indecision.