I just got a 451B a few weeks ago. I a/b'd it with an SM81 at GuitarCenter (before I bought it on ebay), and really was surprised at how much clearer the 451 was -- no contest (it could have been a bad SM81, though). I really went in to buy the SM81. BUT, just to compare, I brought in my MXL603S, and could hardly tell the difference between that and the 451B. So why did I buy the 451B? I really don't f***'n know why!
But, anyway, what the guys above said about one being good enough, where to place it, blah, blah, blah, is something I agree with. However, if you're mixing in the acoustic with a bass, I would position the mike at the 12th fret (or where the neck meets the body), and point it more towards the nut and away from the hole to avoid any bassyness that would conflict with the bass guitar. Just put your headphones on and try a variety of positions.