I agree with MeanEditor, but I also think I know what Migo meant. For 1 lap there's no reason the N20 car wouldn't be faster. For a 20min session, though? Admittedly, I don't have much concept of how long a bottle of N20 will last, but I would think if you're getting on the giggle gas over and over again lap after lap that it wouldn't last very long, particularly if it's triggered off RPM or TPS.
And taking this a bit further off topic
, be advised, MeanEditor, that the overwhelming majority of open track clubs won't allow N20.