This question might get closed out if the TA folks think it's not "travel related." If so, and if you want more info, send me a private message.

I retired four years ago from the USA and spend half the year in BA and half in Punta del Este, Uruguay. I much prefer Uruguay. The country has stable, progressive politics, a stable, growing economy (hardly affected at all by the financial crisis), excellent banks, very nice, well educated people, beautiful beaches along an extensive coastline, and rolling green hills inland. Punta del Este also has a large ex-pat community, more European than American, although the American contingent is growing. (I am married to a woman who grew up in BA and spent 3 months a year in Punta del Este until she was 27, and I speak Spanish, so we do not hang around with the ex-pats, but they are there.) In Punta del Este we lack for nothing that matters (for instance, it has grocery stores that put anything in the USA to shame, fruit and vegetable stands are all over, and there are excellent restaurants and good seafood all year round), and there are many bonuses: four distinct seasons including a winter when the fireplace feels good every day, but no blazing heat, no snow, no freezing weather ever; a short "summer social season" when the rich and famous descend on the area, giving a feel of St. Tropez or Marbella in their good days; both sunrises and sunsets over the water every day (because of the way the area sits on a point dividing the Rio de la Plata from the Atlantic Ocean), and many more. I cannot find a single problem with this area, unless not having access to cheap high definition TV is a problem (and even that is coming). Because it is such a small country, Uruguay does not have an abundance of highly specialized medical care, but basic care seems at least equal to that of the USA and house calls on a moment's notice are still standard. In Punta del Este violent crime is rare.

If you must have a big city, the only reason to choose Montevideo over Buenos Aires is the greater stability of the Uruguayan political system and economy (although that should be, in itself, a huge consideration). As cities go, BA is world class, while Montevideo is a fraction of the size and a bit sleepy.