I had a '24 XC-90 Plus Gas with 21" before getting into a '24 T8 Plus with 20" wheels. It wasn’t night and day IMO, but some incremental improvements on ride quality/softness & road noise. When looking at T8’s, I specifically looked for units with the 20s.
The 21"s look better, of course, but 20"s a bit more practical (particularly in snowly climates)