The bike shop guy is correct, but it only applies to a road trekking crank (Sugino SD600 46/36/26) OR 48/36/24 (110/74BCD). If you are comparing against a mountain triple crank, then you do not have as low a gear as you would with 42/32/22 or 42/32/20, with the latter giving up a low 16-17" gear inches.

Road triple crank; you will loose about 2 gear inches on the top end because of the difference between 48/46 big ring and then 42/40 big ring on the touring double. Not a big deal if you're touring.

Mountain triple crank; you will loose about 1 or 2 low gear inches. 24T on your double (smallest you can fit) as opposed to 22T or 20T on your mountain triple crank.

So in a way, a touring double will give you a comfortable high, medium and low gear from 19" to 100" if you're going with a 42/26 or 42/24 or 40/24 which is a common setup.

First of all, I would not go with a Shimano XT 10 speed setup. The 10 speed XT rear derailleur is setup to work with 10 speed mountain shifters, not 10 speed STI brifters you want to get it to work. You will need a 9 speed rear derailleur like a Deore or a LX with a longer than usual B screw. It seemed my older 9 speed LX derailleur does not need the long B screw as it works beautifully with a 11-36 SLX cassette. For the front, you will only need a Tiagra front double derailleur (10 speed version) as this version will work with the modified 110/74BCD triple crank converted to a double. MAKE SURE you get a good set of 110 BCD ring that's as round and flat as possible or you'll have a slight run out at the extreme 40T/42T to 11T rear or 40T/42T to 36T rear. There is only so much latitude you can get with the Tiagra and this derailleur provides the most widest throw for this setup and works great with a Shimano Tiagra/105 and Ultregra 10 speed brifter.

Incidentally, I do have this setup on my Masi touring bike which was a cross bike with a compact crankset and it works great and you do have to do some double shifting to get some gears, but it only depends on your cadence too. How many gears are you going to use on tour with your cadence? All 20 gears? I highly doubt it. Probably about 8 gears if not less that you'll be using so this double shifting concern is moot. In fact, last year tour, I really enjoyed having a touring double. People with Salsa Vaya 2 have a touring double of 40T/28T and the people I met this year almost all use touring doubles because of the 10 speed cassette. Even the folks pair of Pathlesspedaled who toured with a pair of Bromptons before are now touring with a touring double. No problems. A triple setup is not really necessary at all for touring because most of the time, you are using the 2 inner rings anyhow. But some people need a massive ego boost just to have a 46 or 48T to go bombing down a hill with show and style. If that's you, stick with a triple. If you're practical, the a touring double will do you just fine.