ID4 MkII is using a higher-grade ADC (ES9822PRO these days - I think Julian still measured the older AKM-equipped version, yay for the chip crisis), though the Scarlett is putting up a good fight for having a trusty old CS4272 codec.
Preamp wise, I suspect much the same applies.
Keep in mind that iD4 MkII is "1 mic/line +1 inst (guitar)" in, while 2i2 is "2x mic/line/inst" in, so they are functionally different. The iD4 instrument input is very much an effects thing and not of very much use for other things than a guitar.