I, too, am going to Iceland in September. Another local photographer who has been there suggested that I would want the widest lens I could take. I am going to be shooting 35mm for the most part (But will also have a 645 Fuji with a fixed 60mm.) I have a 28mm, which is approximately equivalent to your 50mm for the Hassie. I'm still undecided about getting either a 24mm or a 20mm, both of which are available for my Minolta AF. But, I digress ...

More to the point, I also shoot an RB67 (NOT going to Iceland due to weight!) For the RB, I have both the 55mm and the 65mm. If I had to choose only one of the two, my personal choice would be to take the 55mm, if for no other reason than it is farther from the "normal" 90mm.