At high loads there's really no comparison: I've had a GTX970 and run a R9 290 (long story, don't ask 😉 ).
Even when heavily loaded the GTX970 produces far less heat, so it's a natural choice for people who want a quiet system, have cases with less than brilliant airflow or which can't handle the bigger coolers the AMD cards demand.
With the changes made with the 3XX cards the heat is better controlled but these cards still demand a case with plenty of well directed airflow if their coolers are to work effectively.
Temperatures with the R9 3XX cards are generally good, the days of the old R9 290/290X running at 95C with a howling reference cooler are long gone and all the cards currently available offer reasonably quiet, reasonably...