There are several different products that carry the name "M3". There are the slot-2 M3 Perfect and M3 Pro products, but I believe what you are looking at is the slot-1 M3 Simply.
The R4 and M3 Simply are indeed identical. In fact, with the first batches of each, you could convert between R4 and M3 Simply by simply soldering or desoldering a few pads on the PCB. That has been fixed with later batches, but the point remains that they are the same.
The R4 is the original product, which the M3 Team buys and rebrands as the M3 Simply. This means that R4 updates come out sooner than M3 Simply ones. Also, the R4 is usually cheaper than the M3 Simply, but not always.
If you're looking for a good seller, I'd recommend RealHotStuff. Their prices are fairly competitive, and they have cheap shipping within the US, and you usually receive your package in about 3 days even with the standard USPS shipping.
Nearly all slot-1 devices have built-in passcard capability, so you don't need to worry about that. The only one that doesn't is the DSLINK (not to be confused with the DSLinker).
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