How was your visit?
Currently I live in nearby Brea. Which is more consistently nice but a little too family oriented for me. I am moving into Fullerton next week. I think you would enjoy Fullerton. During the daytime it can be a little empty but it really picks up at night, many many restaurants and bars and clubs to go to. I find the people do be nice and approachable, it seems moreso than near the beach.
CM does have some night life, and more the beach type crowd. Mostly you'd probably end up going to Balboa in Newport which is great for your age. CM I believe has E&Y and D&T or at least one of those. You wouldnt like the commute to CSUF though.
I'd probably recommend staying in Fullerton and then you can visit and stay with your friend when you go to CM and vice versa

Its just easier to do your daily routine close by particularly when Fullerton does have nightlife its not like a sleepy town. However the other recommendation I would make is to find a couple friends and get a house to rent instead of the apartments. I have never found a nice apartment complex. A house you have more privacy, yard, and its usually nicer. There are some cute older places in Fullerton. I just found a guest house type place on Craigslist for that reason. Walking distance to downtown..
Good luck