It will definitely will not be the only time in Argentina for you after going there. I did a similar 5 day trip April 2009. I did do the 1 1/2 day Iguazu falls sidetrip staying at Sheraton and it was very tight as people have commented. Would have loved to go to Brazilian side too but no time.

No comment on the Cabanas Las Lilas as I didn't try it.