Just got back from Iguazu about two weeks ago. Can't provide a truly objective opinion on your travel options since I've never taken the bus (and likely wouldn't, given my aversion to long drives). I would probably do some shopping around to the US air consolidators, then suck it up and buy the Aerolineas ticket if all else failed.
I definitely WOULD NOT go Southern Winds. Neither of our scheduled flights on SW went off as planned... it seems as if, in its bankruptcy, SW has adopted a "We'll fly when it's convenient for us" attitude; flights are routinely canceled or rescheduled with little or no passenger notice. In our case, things worked out well -- we were rescheduled onto Aerolineas on one leg, and on the other, happened to be lucky enough to arrive to the airport early enough to catch the flight (which was leaving three hours ahead of schedule).
As for entry into the parks and the Sheraton, we had a great experience. When you land at IGR, hire a cab driver to take you to the Sheraton (AR$30, I believe). The driver will very likely offer to take you over to the Brazilian side for the day (We paid AR$150 for the entire day).
The Brazlian park is beautiful, but more appropriate for a half-day excursion; Argentina's side really requires a good seven- or eight-hour effort. For AR$150 -- about $50 -- our driver spent about five hours with us... from the airport to the hotel, waiting for us to get settled at the Sheraton, taking us to Brazil (and handling immigration "formalities" while we waited in the car), waiting for us while we explored the Brazlian park, taking us back to Argentina, exploring Puerto Iguazu a bit (at his suggestion... at that point we were ready to go back to the hotel), and returning us to the Sheraton.
As for the Sheraton, we were completely happy with the accomodations. The hotel looks scruffy from the outside, but our room was pleasant and the staff helpful. The food was also better than advertised (though not spectacular; the free breakfast was the winner here).
In any event, have a terrific time in Iguazu. It's a fantastic sight to behold...