The OP really has decided to take the path of most resistance: An UG is exactly what his business is meant for. If he wants to be a GmbH just for appearance's sake then (a) a one-man band as a GmbH is no more "serious" than a UG (isn't a GmbH minimum 500 employees in any case?), and (b) it's an entity meant for something like Bosch GmbH, not J.Random Dude GmbH.