A book "shotshells & ballistics" by John Taylor has nice detail to answer this question, as it has technical details of every load in production when this book was published.
20ga buckshot loads have a "recoil velocity in an 8lb gun" of 9 to 13 ft/sec.
12ga lead loads have a "recoil velocity in an 8lb gun" of:
10.0 to 12.2 ft/sec for 7/8 oz skeet loads
11.5 to 12.5 ft/sec for 1 oz game loads
12.8 to 15.7 ft/sec for 1 1/8oz loads
14.5 to 17.9 ft/sec for 1 1/4oz loads
16.5 to 19.2 ft/sec for 1 3/8oz loads
Steel loads tend to have higher recoil, as they tend to be faster.
So I would say 20ga Buck is equivalent to most 7/8oz skeet loads and 1oz game loads if the guns weigh the same. However, most 20 gauge guns weigh a bit less than 12ga guns.
You also have some choice in the specific loads you choose. Less payload and lower velocity mean less recoil. Unfortunately, most buckshot loads don't tell you the shot weight, but they do tell you the pellet count. You'd need to look up the typical weight of a pellet in that size of buck and multiply by pellet count to get a total weight. Obviously, if two loads of #3 buck are are 16 pellets for one and 20 for the other, the 16 ball load will most likely recoil less unless it has more velocity than the other load.