Look, the reason a lot of people like the sm7 for rap vocals is because of the nature of what most of us think of when we think of rap.
Now I realize that not all rap artists are the same, and that like anything else, there are going to be different styles and levels of intensity between them and so forth.
However, on those occasions when I am so duly blessed to have the rappers in here for the afternoon (or sometimes well in to the early next morning the way some of them work

) I have almost universally shyed away from the more "pristine" mics with the high-end boost. I like to call them the "sheen" mics. These include the usual suspects from Rode, the Studio Projects C series, and most of the Chinese models, come to think of it. Good for the crooners -- not necessarily for the rappers.
Why? Because generally speaking, they can't hang with the rappers. At least in terms of the ones I'm dealing with, y'all just tend to have a much more dynamic and percussive nature to your vocal presentation.

I have to be ready for very loud passages, softer passages, and very aggressive consonants -- hard "p's" . . . for when you say stuff like "F**k the MF'ing
PO - leese" and stuff like that.
Keep in mind my white ass is trying to be a little humorous here (and doing a bad job of it), but I'm sure you get the idea. Harsh consonants, P's and Esses are things that I have to look out for with the rappers. Not to mention the fact that some of them like to get
right up on the damn mic .
These are all the kinds of things that the sm7 was designed to handle . . . and I can attest that it will be able to handle them for several years worth of vocal abuse.
The
AT4040 is another intriguing option, because from what I hear it's a pretty tough mic . . . I've heard people like to use it a lot on drums. Come to think of it, just about any popular condenser that's used on drums should be able to excell at rap. Think Shure KSM32, AKG C414. And as much as I like the Studio Projects B1, the jury is still out -- I still haven't talked to any of my peers who think of it as anything more than a good, cheap utility mic to have around in case of emergency.
Good luck, and quit losing so much sleep over it.