Gillie, have you found a 21 you're not telling us about?! Don't be holdin' out on us!!
Based on comments from my friend that owned the 21' "Unohu" for over 25 years and made many Bahamas crossings with 23's, I'd say it's no contest - the 21 is definitely the softer riding boat, even though the 23 is generally considered to be very good! He said that every time the seas got over 3-4', the guys with the 23's would be calling him on the radio, "Begging for mercy", asking him to slow down. . . and he didn't run that fast! Even with the V-8, he only cruised at about 20 kts because that only required 9" of manifold vacuum (vs. a "max continuous" limit of 5"), so it was easy on the engine and gave best gas mileage. When he had the I-6 outdrives, he cruised at about 18 kts (regardless of sea state!) at about 7" vacuum. Most guys I know that rode in it, myself included, say its the softest riding boat of that size they'd ever ridden in, although some said that Bob, who was a pretty tough guy and an Army vet that served in both WWII and Korea, just had a higher "threshold of pain" than everyone else!
You have to remember that the 21 was Carl Moesly's original design, and I'm sure he designed it with the intent to race it, to see if it had any weak spots. He had already done that with the American Marc boats before he designed the 21 and started SeaCraft. The 23 is totally a Bill Potter design, not a stretch of the 21 as some folks think; it was never raced that I know of. Potter's background was basically that of a sales guy (who wrote some great ads!) and I don't believe he really understood the VDH design principles. The spray rail on the 23 would make it the drier of the 2 boats, but that's also in the direction to create more lift and a harder ride.
As for which one is more stable in a chop, I'd say that's a toss up. The vertical steps are much taller on the 21 which might tend to act as keels and damp roll, but I'm sure the 23 is much heavier so that might make it better. The difference in vertical CG between an I/O and O/B configuration might be more significant than the hull differences. Denny