The guy at the bike shop would be wrong. With a compact double, you sacrifice either the top or the bottom. You can get the ratios in the middle just fine but you can do the same with a triple. You can go here to do a comparison. Input these values...11,13,15,17,19,21,24,28,32,36...for the cassette. A "trekking" crank will come with 48/36/24 chainwheels and a mountain bike double will have 38/24 chainwheels. A compact double road bike crank will be a 50/34.

The website will let you compare the ratios side-by-side. If you use the mountain bike numbers, you can quickly see that the trekking crank has a wider range and, in my opinion, a more useful range. The compact mountain crank has some large holes that would require constant double shifts to try to get to the range that you may want or need. The road compact double has the same holes with a higher high but the low end isn't as good. With the triple, you have more intermediate choices which come in handy while riding all day.

You can always change the chainwheels but I doubt that you would want to pair a 48 tooth outer with a 24 tooth inner...use the calculator...it would have a good high and an okay low but the jumps would be huge. On the other hand, you could change the inner ring on the trekking set to a 22 (or even a 20) and have a great range.