Originally Posted by pacificcyclist

The gear ratios on the touring double and triple are exactly the same, because it is the CASSETTE that dictates spacing ratio requirements, NOT like some people suggested is the chain ring. What the chain ring does is raises or lower gear inches in a specific ratio for the whole cassette range to meet riding cadence. With a double, you never get exactly the same gear inches as a triple because the chain ring sizes are different except the lowest gear, but the variances are around 10 to 16% for the first 10 high and medium gears. You either spin more or less. These are the same variances found if one owns a mountain bike with a triple crankset 42/32/22 and a carbon road bike with a double 53/39 or a time trail with a 55T/42T and he or she does triathlons on it. Millions of people ride different bikes with different gearing ratios and the differences are not so dramatic that people aren't going to ride them.

What's important in choosing a touring double is your preferred high and preferred low gears and the transition point so the jump from medium to low wouldn't be too dramatic and it seemed, both Shimano and SRAM agreed that around the 40T mark is the sweet spot. I like 42T front, but some people like 40T.