I wouldn't have said this 6 months ago. But I'd go with Android.
It'll be significantly more work porting in the long run. As more and more screens sizes and device profiles are coming out, but I think it's got the weakest developer market with the highest long-term potential earning power. The iPhone market is flooded, so, even if you get your app published to their catalogue, it's still almost impossible to get any kind of exposure.
Android, on the other hand, has huge growth potential and a pretty poorly followed market-place.
Verizon's massive push on the 'Droid' should open that particular device to a huge marketplace. It remains to be seen, however, if and how they'll allow 3rd parties to publish apps to their catalog.
