The Q50 with the 3.7 is probably more reliable and will last longer, but having driven both, the ride quality is lower in the 3.7 than the newer models. I wouldn't buy your current lease model because it's missing too many options.
The newer models have the smaller, higher pressure, more stressed engines but ride better. The interior quality is pretty cheap for all the Q50 models unless you go up the the red sport. I won't buy a Q50 because of the low quality plastic interior, the small over stressed engines, and the lack of features that I would consider to be minimum standard items like memory seats, a decent stereo, elec steering wheel adjustment, cooled front seats, etc. For the price they charge, all of these things should come on every model. For an upscale small car, Acura and Lexus beat Inifniti on reliability, build quality, paint, fit/finish, and comfort.