Which is better? Coke or Pepsi? They're both excellent manufacturers, and they both work fine. Lee sometimes has a better color, sometimes Rosco does, and other days GAM does. I use whichever one looks right...
Their gels are the best (next to ROSCO) in my opinion and they aren't nearly as expensive as ROSCO.
So... If I am buying a gels that I know I will use in just about every show... I will spend the extra dollar and go with ROSCO, but if it a gel that I will only use for one show, then I will just go with GAM. Plus, GAM lets you order directly from them... so you don't have to go through a distrubiter...
I like GAM because I like how the swatchbook is laid out. GAM's numbering system acutely makes sense, unlike LEE or ROSCO, whos numbering system has very little sense.
Gels are fairly cheep, so price isn't an issue, and there gels seem to be good and don't burn out horribly fast, so I am very happy with GAM.
However, it is a coke vs. pepsi type of question. There both good, it just depends whos colors you like better.
Rosco has a warehouse (it might also be their coprerate HQ not sure) here in my town. I take some middle school tech kids on tours through their sometimes and they show us all the products and stuff. They also help us out alot with "give aways" and when they dont give it away i can call and order through any company and pick it up at the roscoloading dock.