The taxi driver who takes you to your hotel will be delighted to pick you up the next day to go to the Argentine side, or anywhere else you want to go. If you can negotiate directly with the driver it will be cheaper than whatever the hotel arranges.
Also, be aware that Iguazu has been experiencing one of the worst droughts in its history, and the falls have been greatly reduced in intensity this year. I was there in July and we weren't able to take the boats to/under the falls because the water level in the river was too low. It's still impressive but not what the guidebooks promise.
If you stay at the Sheraton they will upgrade you to a suite on arrival if you are Sheraton Gold (or higher). The suites have big balconies with a perfect view of the falls.