Hi Olivia,
Brazil is Uruguay's number one trading partner representing almost 21 1/2 percent of the country's overall trade. Conversely the US and UK jointly make up for just a little over 8 percent. Consequently English is not an extremely commercial language for them. The bulk of their trade is with Brazil and other Latin American countries and as such they function quite well in Spanish without any great need for the English language.
Brazil on the other hand is a country with a great demand for ESL teachers and the pay here (while not good) would probably be better than in Uruguay. Most schools as you guessed pay by the number of hours of actual teaching time and their senior teachers generally get most hours, newer teachers getting the left-overs. The money to be made in teaching is in private or "in-company" classes, however the recruiting takes a while.
Is there a particular reason for choosing Uruguay over other Lat-Am countries? You really want to think about some country where English will be a commercial language due to their trade scenario, English isn't something that Latin Americans choose to learn just for fun - they need some motivations.
Cheers,
William James Woodward - Brazil Animator, Expat-blog Team