For a beginner in the sport, I would say fit is more important than any difference in components at that level. The benefit of a proper fit will far outweigh the benefit of tiagra, or even 105. Besides, if you get hooked, you will want better than a tiagra/105 bike before too long (and if you don't get hooked, you'll wonder why you spent the little bit extra). Besides, buying from the local shop creates a relationship that you can fall back on when you need help (maintenance, advice, etc.)