T-Mobile's best complement to the Titan?

[TurboFool]

I'm starting a new job in a couple of weeks and I get a choice of a Blackberry or Windows Mobile-based phone, and the company's network of choice appears to be T-Mobile.

I'm somewhat torn on the Blackberry option, only because I have no experience with Blackberries, so it might be a good opportunity to round out some missing experience, and it could benefit me with the clients who use them. On the flip side I just plain know and like WM, so I'm still leaning toward that.

So what, in your opinion, would be the most complementary choice considering my personal phone is a Sprint Mogul? I know the Wing would be the physically most similar, but on the other hand maybe I should consider a different WM-based model that might give me a different experience? As well, and since PPCGeeks doesn't seem to have much of a focus on this phone, what T-Mobile phone has the most/best customization (ROMs, etc.) options available?