Should I purchase CDMA or GSM iPhone for use in U.S. and Hong Kong/China?

Question

I currently have an unlocked GSM iPhone 4 that I bought in Hong Kong, and have been using it in the US through Straight Talk. I love having the flexibility of an unlocked phone when I travel. Unfortunately, the GSM networks in my area are pretty spotty, and Verizon's CDMA coverage is MUCH better. Not to mention the data speed is really slow.
I'd like to purchase an unlocked iPhone 5s without a contract, and hopefully be able to use it on the CDMA network here in america, while still have GSM available for use when I travel back and forth to Asia.
The pros and cons of GSM vs. CDMA iPhones are still pretty muddy for me, but it sounds like buying a CDMA iphone would be better for my situation because I could utilize both networks when traveling, and yet have the best speed/coverage in my current home in the US.
Straight talk or Page Plus is what I'm looking to for carriers. Verizon's too expensive for me.
Here are my questions:
1. Does my choosing CDMA seem to be the best choice? Or are there any cons regarding purchasing CDMA that I've overlooked? I've heard people say there is no such thing as an unlocked CDMA iphone. My biggest concern is that I arrive in Asia and find that some CDMA lock prevents me from using any other carrier--CDMA or GSM. (Both networks are available there, btw.) I need to make sure I can use data and make calls when I'm there.
2. If a CDMA iphone is the best choice for me, how do I purchase the right one without a contract? Apple store? Verizon? Straight Talk (do they sell CDMA iphones)?Canada? Ebay seems kinda risky to get an iPhone with the specifications I need.
3. Does a CDMA iphone still have the slot for sim cards? That's how I've always switched carriers when in Asia.
Thanks in advance!

Answer 1

Thanks for the reply. I didn't know that StraightTalk disabled their iphone's GSM capabilities. Guess I won't be going there to buy one.
A few follow-up questions...
So is a SIM slot only used for GSM? This is something I never knew. I assumed for any network (GSM or CDMA) that a new sim card was the way you changed numbers/networks.
I've heard people mentioning that they've been able to buy Verizon iphones and use them on PagePlus, but this information was a year or two old, so perhaps things have changed with the newer models/carrier contracts. Do you know of any people who have had success in using a Verizon iPhone 5s on the PagePlus network?