A "Group Poll" is first and foremost about finding a date and/or time. The admin of the group poll can suggest dates with/without specific times that the participants can vote for. Which date/time fits the best for everyone? With a group poll you can easily and quickly find out. If you have connected your calendar under your account settings, and you are the admin of the group poll, the final date will be automatically added to your calendar once you choose the final option.
A "Survey" does not limit you to dates and times but allows you to create a list of text options for the participants to vote for. For example, movies, menus, travel destination, who-brings-what-to-the-next-grill etc. This poll option is also particularly helpful if you wish to create a poll containing days without a specific calendar date e.g Mondays, Tuesdays, Wednesdays etc. The final option of a Survey will not synchronize with any connected calendar.