the honest answer is neither one is better. they are different
gsm uses a sim card which you can go to almost anywhere in the world and go to a local phone shop and purchase a local prepaid sim card and pop it in and your good to go for that area
however you generally get better call quality in the states with cdma as gsm is what is called a hard hand off) what this means is your phone is linked to one tower when you get to far a way it jumps to the next closest tower. this process is when you get dropped calls. the signal either fails to jump properly or just doesnt establish connection in time
where cdma is usually connected to 3 towers at once and kind of "spiders along" through the towers as you go
now verizon when you go out of country gives you a loaner phone which you will have all your calls forwarded too. so its basically the same thing as the other. the only difference is u dont get to use your exact phone over seas so your contacts and what not wont be with you in the phone.
but on the other hand if you have a nice phone (which most of verizons are pretty nice) do you really want your nice phone with you in other countries? or would you rather a loaner should it get lost or stolen? its up to you
i sell cell phones and plans and i prefer verizon(cdma