Well, in the sense of 5-6-7' foot 3ph discs........I don't think there would be a great deal of difference between a tandem and an offset if the weights are close. Take a 7' tandem that weighs 800lbs and a 7' offset that weighs 650lbs and the tandem will out plow the offset any day of the week.
In small utility form, either one will flip dirt one way with the first gang and flip it back with the second gang. It's all about your soil and the weight being applied to it.
Like I said above the real differences don't appear until you get into the full-on ag type discs where the tandem is generally (but not always) configured as a conditioning tool and the offsets are configured for primary tillage..........a nominal 10' towed tandem being lighter while the equivalent offset being heavier built and equipped to handle more weight and use.
In short, don't go out of your way to try and get an offset disc in this class...........find the best, heaviest built of either type that you can get for the money and be happy with it.
JoeinTX, if the offset were to weigh 23% more than the tandem, then which disk would perform best? If they weighed the same, then which type performs best?
Here's a question for anybody that might have an answer. You have two 8' disks, both weigh 1000lbs each, both have 22" notched blades, both have the same angle on the blades. For all intents and purposes they are the same disk. One disk is offset and one is tandem, both are 3ph disks. Which disk works the best, digging deepest, leaving the ground the smoothest, etc?
I do not know these answers, I only have 4',6',8', & 12' offset pull disks. The only thing that I know about 3ph disks is that my neighbor has a 6' tandem 3ph disk that he uses and seems to be content with the 4"-5" depth that it turns over.