I can't speak to the i400 because it's new. But I have an i300 that I'm very happy with (and an i160 before that - also very happy).
It looks like the i400 is essentially similar to the i300 I have.
GPU - RTX 3080, same in both
CPU - i300 has an i9-12900k; i400 has an i7-13700KF. So the i400 has the newest Intel chip, but not the fastest version. That's probably a good thing - it still should be plenty fast but run cooler. I'd be happy with an i7 in my i300.
RAM - 32GB DDR5 in both, but i400 is a bit faster (5600MHz vs 4800MHz)
Storage - NVMe SSD in both, but i300 is bigger (2TB vs 1 TB)
Motherboard, IO, etc the same in both
Apart from the storage, the i400 looks really good. And much cheaper than the i300 was when I bought mine. And I added a SATA SSD to my i300, so I suspect you can do that too - just need a SATA cable (shorter the better - it's a compact build).
I've really liked both my Corsair Ones. Great design and build quality. Fits and looks great on my desk and handles anything I throw at it. If I were buying one today, I'd go with the i400.