If you've only got 100 bucks to spend on a mic for now (and aren't willing to wait for another paycheck or two) I'd get the 57.
No, it's not a condensor, but it's a popular, durable, versatile dynamic that you can always keep in your mic closet.
Think about it: guys with tens of thousands of dollars invested in recording equipment still sometimes find themselves reaching for a 57, because it's the right thing for a particular track. Nobody is reaching for the 990 because it's the "right sound".
As soon as you buy a better mic, your 990 will be relegated to the closet, while a 57 can still be a part of your studio.
Just my opinion.
-Hoax