If you have a mountain bike- you probably have a crankset with 44/32/22 rings fitted and an 11/32 or 34 cassette so your lowest gear could be 22/34. But mountain bikes are heavy and you need those gears on the steep road hills. I know I did. I live in a hilly area and used to ride the Road up the 15% hills and I needed that lowest gear. 4 years ago I went road and got a triple on that 1st road bike. 52/42/30 crank and 12/26 casette. Those hills on the MTB I Struggled up in 22/34 were still hard on the road bike with 30/26----but no harder. They were a lot faster though.
2nd bike and I got a compact crank with 50/34 and 10speed 12/27 cassette. Those hard hills on 30/26 were no harder with 34/27. One years road riding and I had improved a lot so I don't think you opught to worry about a triple on a Roubaix. That depends on your hills though.
I went to the Alpes a few years ago with my first bike. I did adapt the gearing- I replaced the 30t Granny ring with a 28. But these were mountains.
If you are still worried about getting a low enough gear- Then get the Roubaix with a triple---but look at the MTB 10 speed XT or SLX cassette.
http://www.merlincycles.co.uk/Bike+S...-+MTB/list.htm
These are 11/34 but to use them you would have to change the rear derailler and a longer chain. This could be done by the LBS at purchase with some reimbursement for the new parts taken from the bike. You could also look at the SRAM XXX cassette but this is 4 x the price of the Shimano ones.
But to be honest- I think you could get by with just changing the Granny on the 105 crank to a 28t. 28/27 will get me up any mountain and I only do those once a year on holiday.