I too have a CB500X and the parallel twin is much smoother than the NT, and it lugs down to a lower rpm before having to down shift. Max torque and Max HP... meh. I don't operate at those rpms for long periods of time anyway. I want torque and HP at the RPM's I operate at... 3000 to 5000.A V-Twin has a better torque curve (max torque is at a lower rpm than on a parallel twin), peak HP is at a lower rpm, and it is a smoother engine. It doesn't need balance shafts. A parallel twin is also much less expensive to build, hence the reason it is used so often.
Take for example the CB500. That parallel twin engine has peak HP and torque at 9500 RPM and 8000 RPM respectively, while the NT has peaks at 8000 and 6500. I've had both bikes, and the NT is a much more enjoyable engine. The CB engine has to be wound up high to get any life out of it.
My opinion is that the CB500X engine fits the CB500x model better than the NT700 engine fits the NT700 model. The NT could use more low end HP, and maybe a 6th gear. But they're both great rides, and I love having the NT for colder weather riding season... HUGE advantage the NT has over the CB.