I think everyone will tell you that if you can afford it to go with the better components. If you plan to keep the bike any length of time, you'll be thankful for them as you have it for a few years.

I got a Tarmac earlier this year and toyed between 105 and Ultegra components. In the end I splurged for Ultegra, as there were other things involved (better carbon in the frame for one).