I understand why your dad made the decision he made. However, comparing the plow pulling capacity of the 3020 to the 706 was not really a fair comparison. Even though the 706 was only rated about seven hp more than the 3020, the 706 had other advantages. The 706 weighed quite a bit more than the 3020 and the 706 had a 6 cylinder compared to the 3020's four cylinder, giving it anywhere from a 12 to 30 cubic inch advantage, depending on which engine the 706 had (this is comparing diesel to diesel - don't know about gas or L. P.). Additionally, the 706 probably had the TA, which was either much maligned or beloved, depending on who you asked, giving it twice as many gears as the 3020. I'm not making excuses for the 3020. Just pointing out the differences in the two tractors. Even though I much prefer driving the JD, in your dad's position I might have made the same choice.

Full disclosure, I spent most of my teenage years splitting time between a 706 and a JD 4010 so I am not unfamiliar with the tractor and it's capabilities.