If you choose “Guaranteed Winner Selection”, you will receive more reviews as of course, this option will create more attention. You have to select a winner, as long as at least 1 valid review (per review topic) was submitted.
If you choose “Not Guaranteed Winner Selection”, you don’t have to select any winner if no review was useful for you. However, you can select a winner for one review topic, but choose to not select any winner for another review topic. This is completely up to you. You will get the paid reward money back for the review topics for which you didn’t select a winner. Please consider that there is no refund for the $50 listing fee. Please consider as well that the contest page shows your ratio of selected winners/max. possible winners. The higher the ratio of selected winners, the more attractive your ‘not guaranteed’ contest to the crowd.