I'm an ex pat US, live in NZ, but have lived in various countries in the past 20 years. I think you should stick with FTSE, here's why:
If you aren't a yank, dealing with the US tax laws is a royal pain. Also, you have to look at how many times the money you invest is switched back and forth thru the various currencies. That alone could be the deciding factor. If you are investing PS into the US market, then the money is put thru the currency flux twice, once putting it in and once again when you take it out. While if you deal with an exchange that is in your own currency, you don't have that problem.
I realize that in both cases, the actual companies you buy may have various currency transactions but that is a wash, as in both cases you stated, it would be the same no matter what your home currency is.
The other thing to consider is how often are you exposed to the other currency. For example, upon retirement, we plan on living half the time in the States and half the time in Asia/Europe. So I have half of my stock invested in international bonds/stocks. If you don't plan on living in the US or spending money in US dollars, again, it may be better to stick with FTSE.
That's my 2 cents or is it pence?
The question isn't at what age I want to retire, it is at what income. - George Foreman