If the only criterion is routing (and not where you get your frequent flyer points) then I think you are making the wrong choice by choosing STAR over Oneworld.
I don't think any Star Alliance member flies from Quito to the States (not even Mexicana as far as I'm aware and Varig/United/Air Canada certainly don't). Secondly, once you have left your country of origin I didn't think you could return to it on a STAR RTW. So leaving from Miami you couldn't go to Buenos Aires and return to LAX.
Oneworld RTW allows 2 intercontinental arrivals to North America. It would be cheaper starting it in Canada but from MIA you can go MIA-EZE//UIO-DFW-LAX-JFK-LHR etc etc. You are allowed 2 stops in your continent of origin (LAX and JFK) and 3 stops in the other continents.
I presume you are taking an overland trip from Buenos Aires to Quito but even so you have a free allowance of 4 flight segments within South America on Oneworld which would allow all sorts of possibilities including Buenos Aires to Santiago or Lima, Santiago to Quito, Bogota, Caracas, Lima,Rio, Easter Island and the Falklands/Malvinas, Quito to Bogota, or Lima to Iquitos,Cusco or Arequipa on Lan Peru, and of course all the domestic sectors in Chile on Lan Chile.

You pay nothing for date changes and $US75 for routing changes.

[This message has been edited by 3544quebec (edited 02-17-2001).]