I have been to Iceland twice.
There are no guarantees about seeing the northern lights or big blue whales, but one is in winter and the other summer so you won't see them both on one trip.
We went early in June for our honeymoon. We decided not to do the full big blue whale trip as it was so early in the season and dh gets travel sick. We were on a trip with some others that did and didn't get to see any whales. We did do a 3 hour trip from Reykjavik harbour to see minke whales and saw loads.
We also did the golden circle tour (includes gulfoss waterfalls, the geizers, the national park and other stops), a trip up the coast, a trip down the coast and the blue lagoon. The weather was fine as long as you were wearing a coat.
The second time we went in early February. We did a trip to see just gulfoss and the geizers (they were very different in the winter), and a trip to see the northern lights (there was a good showing). It was absolutely freezing even wearing thermals and multiple layers. The bus driver looked at us as if we were mad when we asked for two adult tickets to the zoo. There were no problems with crowds though as we were the only ones there!
Whatelsewouldibecalled - the showing we saw the northern lights were green and dancing.
I think you will have to decide what you want to see/do and use that to determine the time of year that you go.
We hope to go back in a few years with out daughter.