Power-wise there are some differences. The 350U was the earliest of those three, and I believe came out in 1956. It actually had the most HP at 39 and was, I believe, and outgrowth of the Super H/300/350 tractor. The 240U had 28 HP with the C123 engine, and was an outgrowth of the Super C/200/230 tractor. The 340U had 32 HP and was a new (1958) design entirely, with a new C135 engine. I believe the 340's frame was the same as the 350. As you can see, the power between the 240 and 340 was not a major difference. However the 340 made available more advanced options, including T/A and live hydraulics and PTO. The 340 is also considerable heavier, and has a much more heavy duty power steering system. The 240's power steering is a Char-Lynn motor, which is more of an assist.