Upload & Sell: Off
Got both on my 7D (crop like your 60D). Personally, and as a lot of person already mentionned, I will not use the 24-105 if it is my widest option. So as a result I will go with the 15-85 even if its construction is not on par with the 24-105.
If you go with only two lenses for what looks like the trip of a lifetime, I would change your 55-250 for a better (and unfortunately more $) telephoto: 70-200Lf4 IS (with maybe a used 1.4x extender) or the new 70-300L IS come first to my mind. IQ of the 55-250mm is not reporting to be good (report here) and since you enjoy telephoto, your expectation should be higher.
For your information here are my "travel" kit:
Europe/City: 10-22, 24-105, 70-200L f4 IS
National Park: 10-22, 24-105, 100-400 (if wildlife) 70-200 (otherwise)
Backpacking (4~5 days with tent, food and bear box in my backpack): 10-22, 70-200L f4 IS
If, for any reasons, I am limited to 2 lenses for cityscape, I would replace my 10-22 + 24-105 combo by the 15-85
PM me if needed more information