Well, if you go to T-Mobile now you'll only get 500mb of data, not sure how much you're on now.
Why would you go to a DHD before rooting? What does the DHD offer that makes you want to go to that before rooting?
I have no experience of the DHD so I can't comment on it but the Desire is an awesome phone and rooting makes it lots better, I would root first and see what you think.