Wow, there are quite a few people posting assumptions and not stating them as such. A lot of mis-information is being passed around.
I'm not stating that my knowledge is the absolute truth, however, working at a bike shop, at lot of the knowledge I've learned is from ex-racers, and guys who have been around bikes for 30+ years.
The reason why sora and tiagra wear faster than 105 and above is because of the plastic parts. They aren't built with the same quality materials as 105 and above, and thus, they wear faster. That's it. Corners are cut to make them cheaper, and the cheaper plastic parts wear faster than metal. 105 and above are all metal, and thus last longer. Sora and Tiagra are completely functional, and capable, however, if you're doing more than 100 miles a week, I'd recommend 105 or above for the longevity of the parts.
Aside from weight between 105, Ultegra and DA, the higher up you go, the more precise (see: smoother) the shifting is. The difference between a properly adjusted 105 front derailer/shifter and an Ultegra is almost night and day (IMO), especially when going from the smaller cog to the larger. The difference between Ultegra and DA isn't nearly as great. The jump between Ultegra and DA leads to parts being lighter, and ultimately, they wear faster. Although its not a huge difference in terms of longevity, DA will wear faster than Ultegra (according to the shop mechanics). Generally, unless people want to show off their bikes at starbucks, or are racers, I recommend Ultegra simply because of the value you get over DA. Unless you're a racer, and grams actually matter to you, you're buying the DA name, and that's it.
If you're familiar with the shimano MTB groupo's, the jump between XT and XTR is similar to the Ultegra and DA. Lighter, but not necessarily stronger.
