The UI of your start-up very rarely makes or breaks it if you iterate and keep it clean. This is a great article about increasing user retention from x% to x+y%, but like A/B testing and everything else, it will not make a bad product good or a good product bad. Just keep moving forward and listening to your users, they'll tell you what they want and then once you have a customer B, you can A/B test to your heart's content.