It's basically up to you, but Hildon is what has been used for the system applications, so it is very stable. Qt on Fremantle has yet to really prove itself, so you may run into some issues. Also, I believe that the Fremantle specific Qt additions aren't well documented yet.

If you only care about the N900 and want to make a standard application that behaves like other default applications, then I suggest Hildon. If you want to port your application in the future to other Nokia devices, or you are planning to make a custom UI anyway, then Qt is worth a closer look.