If my .17 Rem doesnt work for a fox/coyote, as a fur friendly round, if i had to stick with a factory caliber, Id say the 221 would be in the top 2 or 3. If I was just hunting fox hunting with this rifle I can think of many factory, and nonfactory options that would be better. As an all round predator/varmint rifle, again in the top 3.(factory)
But to your original Question. If your refuring to a caliber being fur friendly, id imagin the 221 being a better choice than the 223. But with the factory loadings available with the 223 it may be better to just stick with the 223 and get a 22mag or 22hornet if just for fox with the extra $. Also you cant beat a 12ga. for fox either.
!!!DISCLAIMER!!!
I have never hunted with a 221, nor does anyone i predator hunt with use one. This advice ive given you isnt thru personal experiance. Just a year and a half of research of factory, and custom calibers to find the best caliber for hunting fox, and coyotes in Pennsylvania and still be fur friendly. I am still searching.
The .17 rem gets my nod for best factory caliber, if you reload (my opinion) I'll be running it thru the tests this season, and will post my results.
For my kind of hunting... 50% fox 50% coyote (1% if i draw a bobcat tag) fur friendly , and within 200 yards id consider the 221 a better choice than the 223, but messy on fox with full loads as the others have sugested.