Online surveys remove the need for data entry before analysis. However, they tend to have lower response rates than those delivered at the time of the event. This can be because people forget about the survey after leaving the event, or because it requires extra action compared to completing one at the time.
Response rates can be increased by making the survey smartphone-friendly, or providing tablet computers so that people can complete the survey at the event.
If your event has an online registration process, you could collect some baseline data before people attend. For example, you could ask registrants to rate their knowledge on a particular topic, and compare the responses to those given after the event.