Of course your mic selection will need to change completely after you get some room treatment isntalled......
(just kidding.....I couldn't stand the fact that nobody brought it up yet! LOL!)
I rented an MD421 once, I would describe it as the only Sennheiser mic I've ever wanted to keep. Unfortunately, I did not own my SM7b yet, so I could not tell you side by side comparisons.
I got lucky with a little Soundcraft Compact 4 mixer that has phantom power, etc, for $99 bucks (might be higher now). I was surprised at how quiet it runs at near full gain, and at how little it seemed to color the sound of the mics. Plenty of gain for my SM7b, very natural sounding, might give you pause before buying the M-Audio thing, if you are concerned about hiss at higher gain settings. I found the SC4 20 minutes after Guitar Center told me they were out of the M-Audio units, or else I would now be whining about M-Audio hiss myself.
Without being able to speak to the MD421 (I do want one, but haven't been able to justify it yet), the SM7b is quite versatile. Used it on loud rock vocals, great. But so far I've not found any other dynamic that sounds as good on my guitar cab...... except a Heil.
Me thinks Heil mics are too often over looked.
The PR20 is a great cab mic to have ina pinch, a far cry better than any SM57. The PR30 and PR40 are the most condensor-like dynamic mics I've ever heard in my little basement flop, very similar to the AT4033. For the money, nobody should over look the Heil mics. They have some great testimonials from national brand users, and I know from home use that the hype is apparently justified. I dont regret selling very many mics, most I'm glad to be rid of, but I do regret selling the Heils and the Cascade Fathead. I am aching to try the Lundahl mod Fathead. And I cant wait to nab another PR40.
Which leads me to my other point, AT40-something mic is a great first condensor. Competitive price, and noticeably better sound than any of the Chinese budget condensors. I can say I am happy that I let go of all Chinese made mics in my possession.
I like Richards mention of using an AKG on the rear of an open back cab. I tried that, works great, sounds cool if you phase it and mix it accordingly, except I used
a D770, also tried it with an EV ND468 which is another good cab mic even though it has a very characteristic EV sound like all EV mics do.
All that said, I voted for the Cascade Fathead in your other thread poll. I absolutely hate that high freq edge or presence peak in my guitar sound, and I know from experience that the ribbon will tame that harsh edge, even if it is a budget ribbon. Not my first choice, but of the two options in your poll I would take the ribbon for a guitar cab. If you are going to rely on it for everything else also, then get the 421.