Per the Big Finish podcast, while Bradley hasn't quit or anything and may return for special occasions, Noonan is the focus going forward. I would guess that Bradley might be a bit expensive for BF to use regularly in an era where they're apparently going to treat every Doctor they have access to equally, regardless of whether they have any original cast members to work with. Plus he's still a busy actor in a way that other BF Doctors (Tennant and Eccleston to the side) aren't.
Noonan is pretty plainly doing an impression of Hartnell; the announcement and podcast make it clear that he put a lot of effort into learning about and sounding like Hartnell. I personally think (based on the limited sample available, but again it's what they chose to release with the announcement) that it's a pretty bad impression, focusing on high-pitched tremulousness and ignoring everything else about Hartnell, but it's still an impression. If they were going in a more original direction, I imagine they would have looked for a better known performer for it.
