Buy or rent a telephoto lens for my 60D

I'm going to Alaska in early April to (hopefully) see the northern lights / whale watch / experience the utter awesomeness of Alaska. Right now I don't have a telephoto lens and honestly I don't really feel the need to have one most of the time. When I go hiking, mtn biking, etc, I don't want to lug a lot of gear so I just take my RX100 along. In town my X100 is the go-to. But I DO want a telephoto for this trip, especially for whale watching and for the train ride that goes past Denali.

So should I buy a 75-300mm ($150) or 70-300 ($500), or rent a 100-400 ($78/week)?