R4DS is the best Slot 1 Flash Cart.
I did say that they were exactly the same thing, and they are. The difference is in firmware updates. R4 Adapters regularly releases firmware updates to address issues that arise. M3 has made R4DS firmware incompatible with the M3 Simply, even though the hardware is almost exactly alike.
This means that after R4 Adapters has released a firmware update, you have to wait something like a month or more for M3 to convert the update into something an M3 Simply can use.