I think there's a couple of different versions of the JM47 ... but don't quote me on that. Either way, it's pretty much the same as
the SE Electronics SE2200 I have. It's definitely good value for money, and a good starter mic if you're wanting to play with that 'condenser sound'. My SE2200 has been used on acoustics, amps, vocals and piano and done a surprisingly good job on all of them.
I haven't used
the AT4040 (although I have a few of its stable-mates) but here's the scoop on what people round here reckon of it. It's very detailed and transparent for such an inexpensive mic, and that would be why it sticks around. I wouldn't buy it expecting you to do many favours - if your guitar or voice is crap, it won't help you out, but if they're great, it won't get in the way either.
I would tend to agree with Chessrock that if I could only have one, I'd have the AT. But there's nothing wrong with the JM/SE if you want to pick up something and learn how to use it.
So I guess it depends where you're coming from - and the other big factor is the quality of your room ... it will affect both mics but my experience is in my rooms is that the accurate mics let me hear stuff I don't want to, whereas coloured ones (and dynamics in particular) let me take the emphasis off bits I don't like. But either way you should look at how your acoustic space is set up.