There's only maybe a $300 difference in the cost between the two, but the 105's reliability is far better and so is the operational aspects. HOWEVER, if you don't ride much and don't ever plan on racing, or are on a real strict budget the Tiagra is a good enough groupset for casual riders, or tight budgets.

But keep in mind, on a factory build it's really cheap to get groupsets from the factory vs buying them by themselves, you may only have to spend an additional $100 or so to get the 105 from the factory. If your going to be using the bike for the long haul and will ride it frequently then if at all possible get the 105, otherwise the Tiagra is fine.