neither..if you are starting out, I would get a sub $200 motor like the Novarossi BF21. Nova's (IMHO) are the easiest to tune. I run a 21K in my truggy and the BF21 in my buggy. My 21K has about 6 gals on it and plenty of pinch left. I expect 4-5 more gallons.
my BF21 is fairly new, less than a gallon and has not opened up yet but has plently of power.
my next motor will be the P5 to replace the 21K.
people swear by OS but I have seen to many failures from others with them even after 2 Gal. I know nothing of the Ninja