3-speed

Here's my 2 centavos.
I have owned and driven both for several miles in different cars over several years. (Mostly '69 to '76 A-Bodies)
The 4 speed is a stronger transmission for sure, but the 3 speed that you have isn't all that bad unless you really abuse it.
The transmissions in the slant 6 ''three in the tree'' cars are the ones that are fairly weak.
The three speed that you have is a 1:1 ratio in 3rd gear just like the 4 speed is 1:1 in 4th. If you feel that it needs an extra gear, it is more the product of your rear gear ratio and tire size. Back in the days when your car was new, that's just the way it was and you just lived with it.
Driving experience of both is pretty good.
The 3 speed has a bigger spread between gears by design, and the 4 speed is a closer ratio between gears of course.
I think the whole idea makes sense that the 4 speed would have less R.P.M. drop between the gears, and that would make it better for racing as well.
For general driving, the 3 speed is just fine, with the added bonus of less gear changes to get you where you want to go with a little less effort.
Also, i do buy the reasoning behind tricking the insurance companies by offering the 3 speed over a 4 speed, but it also was to save money and lower the price of the car as well. They offered the 3 speed floor shift as the standard transmission for '70 Road Runners with the H.P. 383, so i doubt it was that weak, and it's the same basic transmission that came in the 340 cars.
My old '71 Demon had a ''3 by the knee'' transmission in it and it took a LOT of abuse without breaking.
I thought it was a great package, and was fun to drive.
BTW, your car is fairly rare, they made 2,401 of them, and the 4 speeds were slightly more at 2,871. Automatics were 8,530.