Most likely the nitro.  You get the most benefit from the initial percentages of nitro.  5% is much better than 0%,  10% is better than 5%, 15% is better than 10%, etc.  When you get up to 25% or so the improvement in power gets less and less with each addition.  In addition to more power, more nitro gives the fuel a bit broader ignition range, so it tends to run smoother and it more tolerant of how the motor is set up.  0% nitro requires some pretty specific tuning of the motor to get it to run right- exactly the right squish band clearance, closer control of the head volume, the correct plug.