Hi Denise
I've been to both Norway and Iceland. Have a look in the review section of the forum for my review of Norway and I think others have done reviews of Iceland.
Of course seeing the Northern Lights is not guaranteed - I myself failed on three previous occasions at different times of the year and in different countries! Eventually saw them in Finland last year in August, Churchill (Canada) in October and Tromso (Norway) in February of this year!
So, which country to visit, is entirely up to you. You can always do the other another time....
Regards
Sylvia