The above sites have different but complimentary functions. Both deal with conferences or meeting situations where feedback from the meeting participants are useful to the teacher/lecturer/speaker.
There is a reduced rate for teachers for Sli-do and Meetoo looks free or similar discount for teaching situations. If your participants want to ask a question Sli.do puts the question up on screen. You join a meeting via the Sli-do site and hashtag the meeting address.
If you want to survey your audienmce than Meetoo offers a simple and effective way to poll your audience. I found both these websites simple and easy to use both as a participant and a teacher creating them.
I intend to try them out with my university students next year.