Very different mics. The M179 is probably one of the most versatile mics you can get under $500, has continuously variable patterns, an exceptionally low self-noise level and exceptionally wide frequency response, and notable for accurate, neutral response---which may or may not be what you need in a vocal mic. Personally, one of the reasons I've always liked it is because it has a kind of dynamic mic quality where other mikes get brittle or hyped. It's more expensive than it used to be but still a bargain for what you get. To me, comparing the M179 and the 990 is like comparing apples and oranges---they are very different (and I do have both).
Paj
8^)