this topic is so touchy. i think its more a point of personal preference. i had a 7.5lb fidanza flywheel on the first car i ever drove and now i have a fidanza on the car that i have now. so, it doesnt bother me that i have to give it more gas to "go" and to tell you the truth, i HAAAAAAAAAAAATE driving cars with stock flywheels. and its one of the cheapest flywheels you can get. quality, its fine. your never going to break it. if you do, you must be doing something stupid or be throwing out some ungodly ammount of power... but its just an opinion of personal choice. i also use my car for autox, so the lighter flywheel benefits me even more because of the type of racing i do. as for gas mileage, you dont really notice any difference. its all about how you drive.
you gotta rev the engine a little higher by like.. idk, 500rpm with the lighter flywheel and let off the clutch faster... its not that difficult. but then again, i grew up driving a car with a lighter flywheel... you just gotta ask yourself what type of driving you will be doing with the car, and how much you wana spend... after you figure that out, youll make the right decision