Generly the 12 ga has more "punch" due to a larger diameter and more weight. Some 20 ga sabots appear to hit harder than some 12 ga sabots due to sabot shape and/or construction. Same goes for trajectory, some 12 ga loads will be flatter than many 20 ga loads though I feel it is not enough to matter.
I shoot both 12 and 20 ga guns and found the key is to use the brand of sabot that the particular gun likes best. The fastest, hardest hitting sabot on the market is not a good choice if your gun shoots patterns rather than groups with it. My most accurate slug gun happens to be a 20 ga but the sabots that are the most accurate also seem to expand minimally based on internal damage and the distance I can expect the deer to run. I only use that gun when I am certain I others are unlikely to be around. If there may be others in The woods then I use a 12 ga as the larger slug seems to drop the deer quicker as it is of solid lead rather than copper.
Buy the gun you wish and use whatever sabot is most accurate in the gun. Realize that the combo of ammo, shotgun, and shooter may not be up to your desired maximum and work to get within range. If 200 yards is mandatory then I would look at an inline muzzleloader as that would be a more likely choice.
