Here's how they sort out, roughly:
The AT831b is a lav ("lavalier") mic intended mainly for vocals, I think. It has a bump in the high frequencies to accentuate clarity when used in typical lav mic locations away from the speaker's mouth. I don't think that's ideal for an acoustic instrument, so it wouldn't be my first choice here.
The Pro 70 is another lav mic, and it has a flatter frequency response (no bumped highs). It comes with a battery case that can provide power for the mic, or it can run on phantom without a battery. One advantage with this mic is that you could run the signal into something like an acoustic amp input that didn't supply phantom power, although you'd still need a good preamp because the box only supplies bias voltage to the mic. The battery box isn't a preamp like the one that comes with the K&K Silver Bullet (check out that mic too, while you're shopping).
The ATM350 and Pro 35 are intended for instruments, and both have a fairly flat frequency response. The ATM350 sounds a little better than the Pro 35 -- a bit more detailed and smoother-sounding, but it's a subtle difference. I'm using an ATM350 on my Dobro and I'm happy with it. I use a DPA 4099 on my mandolin and guitar, and that's also the mic used by the guitarist and fiddler in our trio. I think the 4099 is the best clip-on mic out there, with the best mounting system, but people tend to freak out when they see the price.
Anyway, back to these AT mics. The mounting systems are different, which might influence your choice since you're planning on swapping between instruments. The Pro 70 and AT831b come with a clothing clip and a flat edge-mount clip for guitar soundholes. The gooseneck mount supplied with the ATM350 and Pro 35 is ideal for mounting on a mandolin if you have a Tonegard attached. It works great on my Dobro, clipped to the tailpiece with the mic pointing towards the coverplate. However, there isn't a convenient attachment point for that kind of mount on an acoustic guitar. Clipping it to an upper strap button on the neck might work. It might even be possible to use the guitar strap as a mounting point, if you can still get the mic close enough to the guitar for good feedback control.
Good luck with the project!
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