Use the one with "Original" on the box. As far as formaldehyde, I didn't know it contained that. I'm in now way a chemist, or even close, but doesn't formaldehyde evaporate when exposed to air? (I don't know this, and I'm truly asking this to find out myself) I know in the liquid state it has a pretty potent smell to it, and there are no signs of that smell in Magic Erasers.
I always rinse them real well prior to use, and have never had even a minor issue. That review bags on them saying they are very abrasive and can take paint off walls. Yup, i agree. If you have old crummy flat paint on your walls and scrub it with a magic eraser, it's gonna take paint off. But then again, so will a coarse sponge, a Scothbright sponge, and even a cotton towel. Magic Erasers are in fact a micro-abrasive sponge. In my experience, they are much less likely to scratch your acrylic than even the plastic scrapers made especially for acrylic by Kent Marine.