I say stick with the Evo. I owned a 370z...after 8 months i sold it. Cant say i miss it either.
Now the main question is...do you owe anything on the car. What year is it? Take it from me, you dont want negitive equity coming over if you trade.
Now to get into the whole 370z vs Evo....
Gas mileage: not sure why people are saying the Z gets better...i was averaging about 13-15/100km in the Z. I'm at 10.7L/100km with the Evo.
Winter/Weather driving: Well it doesnt take much IQ to realise AWD beats RWD
Performance: Evo takes it again. Sure the Z has abit more hp, but the Evo will beat it on the 1/4 mile and road course.
Ride quality: yea ill admit that the 370z had better quality for DD. What you gotta remember about the rough ride in the Evo is, its ment to shine on a track. Take your car to a road course and you will see how she handles and how it shines. Also the clutch chatter in the Z is retarded.
Looks: well having 4 doors is a plus...get better performance and yet you get 4 doors. I'm half and half on which car looks better.
Now to top it off...the S-AWC system in the Evo will not only help you on the track (if you end up racing) but this thing will save your *** on the streets if something comes up. And it doesnt effect the power output like the Z's VDC system does.
This was me the other day at the track. Came around the turn and one of my tires clipped the edge of the track and i lost traction. The system in the car helped me get straight again, even though i turned the traction control off (one push)
EvoX @ Stratotech - YouTube
Also the Z holds a HORRIBLE resale value....the Evo doesnt (hence why used evo's are like 30K)