Probably over a hundred countries offer CDMA. No doubt, GSM is a larger network worldwide. Which is better? That will be primarily personal choices and the customer service, reputation and network offered by the local carriers of each system will be an important feature.
Personally for me, CDMA is the best since it is offered by Verizon. I have had great service with Verizon, they do have the largest network in the USA, but more importantly, where I live, there are only two companies that offer full 3G coverage; Verizon or Sprint and Verizon's coverage area is five times larger than Sprint. AT&T only offers EDGE in this area and the network is small. (On a side note that is probably not a fair example since it is only me, I did have AT&T as an issued phone for six months and I have to say everyone mentioned it sounded like I was in a trash can speaking. I would say it was the phone, but I can usually tell when people are using GSM to call me.)
But for others, in other places or for other reasons, a GSM carrier such as AT&T is the best.
For overseas, it all depends on which country you will be in. For the USA, a lot will depend on where in the USA you will be living.
I do not believe that one is actually better than the other, it is all up to the provider to provide the coverage for either system.