iPhone for BOTH US & CHINA

The iPhone is made in 8 different models. Each has functionality in different regions of the world for different carriers.

I live in China but travel often for extended periods to the US and other countries. I am planning on replacing my 4S with an iPhone 6 if I can use it easily (sim swap) as I move between countries.

After reading the different frequencies offered for each of the 8 models there is not one that is ideal for both the US and China. If you want to use China Mobile's new 4G service you must buy their model 1518 but it offers no FD-LTE for use in the US. Also, it can not be purchased in the US so one must pay a premium to buy it in Hong Kong or in China where the prices are $200 higher!

Are there serious technical difficulties in providing more frequencies for a universal (world) iPhone that easily functions in both China and the US on the fastest networks?

Why do they have so many models?

Will the iPhone 6 resolve this?
I don't want to be forced to use slower technology when I am paying top dollar for a premium phone! Do I have to sacrifice functionality when I go to another country?

China's internet is generally slow and one has to use a VPN. I'm tired of slow speeds but I want a phone that serves my needs in many countries. Is that too much to ask?