Ask HN: Learning phone development - iPhone or Windows 7 Phone?

I've put it off for far too long and I've decided I want to learn how to make phone apps. Now, the question is: should I learn for windows phone or iphone?

I'm interested in your thoughts on (a) what's easier/quicker to learn, (b) which will be more useful in the future, (c) which will offer a greater chance for earning income, and (d) whatever you thin is good advice...