SDexpress continuous write speed will probably be as fast as any of your CFexpress cards or when Xsfering to a PC. Canon is on the memory card committee. They are fully aware of what will be happening in the memory card arena. There is only 4 manufacturer cards compatible and tested with Canon's CFexpress bus for high resolution video that wont give video recording flags. Basically a crap shoot. And they arent sold locally at Best Buy either
And lets not even debate the card reader crap. Companies struggling to sell cameras and then consumers have to search for card readers.
While the SD Express 8.0 standard is just a bit slower (just under 4GB/s) than the current CF Express hardware in the field, that standard was only finalized in May; the 7.0 standard for which new hardware is just coming out is 1/4 that speed. Canon probably can't yet access sufficient hardware for 7.0 if they wanted to.
Further, no one is talking SD Express on this camera, just SD. And on previous amateur/pro transitional cameras, Canon dumbed SD down with earlier-generation SD controllers. Basically a nod to people who pick up a used body and sabotage themselves by buying only crappy off-brand SD media--the same folks who visit forums and complain their camera isn't fast enough, and it wrecks their cards, and they bought a card reader but that didn't work well either, and they want Canon to make a better camera with features beyond the current technology but sell it new for $17, and... ::whining continues as I walk away::
Multiple cards of the same brand, type, and capacity are easier to manage with multiple bodies, and dual write is faster because they share the controller (probably slightly less heat generation too). I currently don't give a crap about card readers: I'll download while I eat dinner anyway and a high-end camera connects directly to a network (as this one does via Wifi 5G) or to a computer, or both. I don't give a crap about shitty off-brand cards of any type, and buy from the few reliable manufacturers who test their hardware prior to shipping. I'm not going to risk a shoot for the sake of a few pennies. I also am not going to overpay by driving to a Best Buy: Adorama prices lower, buys their cards directly from the manufacturers instead of from Sticky Louie in an alley, supports professionals, and ships free to my door.
If Canon eventually put ultra-fast dual SD Express 8.0 in, that might be fine too. I don't see that possibility very soon as it is years until that hardware is delivered, and because Canon is a heavy promoter of CF Express. In the meantime, I would rather see a camera designed for the amateur/pro transitional market (like the Canon R3) outfitted with hardware reflecting its target market, namely, dual CF Express slots.