$50 seems like the better plan to me. Ofcourse, if I ask the users who haven't subscribed and still use the free model, they think $50 is too much and they would be more likely to pay $20
Of course they would rather pay less! But you also have to consider that when people start paying, they usually expect a higher level of service. If you can keep the number of people with high expectations to a minimum and still turn a profit, you'll be in better shape.
Start with the higher price, and see how it goes. If you aren't getting enough customers, you can always offer limited-time discounts (e.g. "Regularly $50/yr, but sign up during December and get your account at $40/yr for life!").