The hotel situation is a little awkward, Mobri. There's only one place to stay in or near the park - the Sheraton, which is comfortable, like a reasonable business-class hotel, and has distant views of the falls. You can have a leisurely breakfast on the terrace, and then walk through the hotel gardens down to the falls before the tour buses arrive. It's a perfect place for exploring the falls by day. It also has a great pool and terrific spa with hydro circuit, etc. It's quite expensive, but you can usually find deals on hotels.com and the like that lower the cost a little.
The problem comes when the sun sets and the park closes, and you're stuck there, with only the (poor and expensive) hotel restaurants and your room TV for entertainment. The town is a 30-minute, 100+ peso taxi ride each way.
There are lots of hotel options in the town, from hostels and family cabañas to the splendid Iguazu Grande Hotel, which is by far the best lodging in the area.
If you stay in town, you have shopping, cafes, good restaurants, and even some clubs, but you're a long way from the falls. You need to take a bus or taxi out there, pay an entrance fee to the park, and join the hoards of others who arrive at the same time and try to see everything in time to catch the bus back into town.
When we visited the cataratas a couple years ago, we ponied up the price of the Sheraton for 2 nights and really enjoyed being able to explore the park in depth, on our own schedule. After checking out the hotel restaurant, we decided to call a taxi to go into town for dinner. The second night we stayed in the hotel. We took a thalassotherapy circuit to calm the sore muscles, had drinks by the pool, and treated ourselves to the hotel's outrageously expensive and thoroughly mediocre buffet dinner, where we overheard other guests discussing the Aerolineas strike.
Unable to book other flights the next morning, we checked out the last corners of the cataratas park before moving to the Iguazu Grand for the night. It's much more comfortable, with better food and a nice casino, plus it's within walking distance of the town center as well as the duty-free shops on the Brazilian border. But if we had checked in there on arrival, I doubt we would have had the same experience of the falls that we had at the Sheraton.
We lucked out with a LAN flight back the next day.
We stayed in the hotel in the park at the brazilian side. (Forgot the name- but there are only 2 hotels in the park). I would recommend it, not because it is a nice hotel (it isn´t) but because of its location. You hear the falls from your room, and you must just cross the street to see them. After the park has closed, you can still have a walk at the Brasilian side (the views are splendid!!). If you are an early bird, you can do the walk another time before breakfast. I think the walk is so nice we did it 2 to 3 times a day!!!
If you stay here, you must surely do the Argentine side too, as that side is completely different. It may be a fuss, crossing the boarder each time, our passports were full of stamps after 3 days stay, but I think it is defenitely worth it!
There are a lot of nice hotels outside the park, you will probably enjoy the hotel more, but see less of the falls...
Just in case you didn't know: If you book your hotel on the Brazilian side and you carry a passport from the U.S., Canada, and/or Australia (to name a few), you will need a visa from the Consulate General of Brazil that serves the area in which you live.