Mark,
Couldn't agree more. Quality beats quantity. In fact, I would say that RSS or Email subscribers are even more valuable than Twitter followers, especially email subscribers.
Why? Becuase email subscribers (double opt-in of course) not only want to get your message, its delivered to them. They don't need to access an app that may or may not be central to their lives. Email is central to everything.
I'd take the million bucks as well. That said, I do think the Twitter spot has value, just not the same as $1M cash.
If you're a NGO that looks for individual donations, it could be worth a million dollars in donations over time - with an investment of your own resources (staff, supporter attention, etc). Maybe $200K of investment over 3 years to make that spot generate 1M in 3-5 years - contingent on the right emotional sell.
If you're not looking for individual donation, nor need mass volunteers, the spot is much less valuable. So the $1M now would go a bit farther.