250r or 150rb? Don't know what to choose?

Well i went and sat on a 150 rb today and it felt pretty good. I am 15 5,8/5,9 and weigh 145 pounds. I have looked it up alot and many people said a 250r would be better but im not sure. Before i owned a little 80f from when i was 12 till know so i am used to the small size of the bike. I know i could get used to the 250r after awhile but just looking at it, it looked to big for me. Since i am so used to the small bike any of the bigger bikes just look to much to handle for me. I am also an intermediate rider and want to start motocross.