Wyll definitely benefits more from medium than Astarion does. Wyll with mage armor has AC of 14. Wyll with moderately armored + shield + best medium armor gets to 19 AC. This is a huge difference.
Astarion with mage armor has 16 AC. Give him +1 dex with an ASI and he goes to 17. You could give him moderately armored and also get to 19 AC, but the improvement is minor, and you have to use a shield to get there (w/o shield moderately armored does NOT improve his AC at all relative to mage armor, and gives only a 1 point bonus over studded leather).
