@nyCecilia
If you do not read Magazines or a lot of PDF devices, stay away from LCD devices. If all you need is read a simple book, any eink device will help on that matter
A classic looks the same on color or black and white.
eink devices are lighter and more portable than regular tablets and usually give you weeks or even months of battery with a single charge!
Yes, Kindle 3G is International, Nook it is not. You can check Kindle 3G coverage on Amazon website but I think there is in Mexico or most important areas there. I don't use it frequently but Kindle 3 has a browser so will allow you to check your email; very basic interface but better than nothing.
Nook touch does not have a browser and it's wifi only. I don't remember about Sony, but I think there is just one model with wifi capabilities and not sure if has a browser.