The heat treated one is noticeably stiffer. The non-treated one is rather springy/flexy. It's definitely worth getting the heat treated one. They only come nowadays in 42's. At one point they were made in 40's but they were offered in a thinner tubing that tended to crack. The Nitto catalog only lists the 42's for the past 2-3 years.
These are great bars. Many more alternatives for hand positions, plus they take brakes really well (cyclocross top levers, regular road brake levers, or aero time trial bar levers).
I have an extra pair from a frame I just sold if you're interested. Mint/unused. Feel free to PM me.