Well, as you put it yourself - in l-a-r-g-e-r cities mobile phone coverage more often than not is not an issue. If you wanna go cheap with text messages - knock yourself out, in these cases, by all means, go by GSM....

BUT: should you chose to leave the beaten track (and on your bilke you´re most likely to do so), be assured that in large areas of the southamerican continent, no GSM coverage is available!!

So if you want to go safe ( I mean the safety of making a phone call when a situation really calls for it) there are only satellite phones to provide that kind of service.

It all depends on where you put your emphasis...