I have been playing around with disk brakes on my converted MTB and there is no comparison in the rim brake arena. The disk system is capable of lock the wheel at any speed, but has such a smooth modulation that it rarely happens. Reading all the info on the 105 group bringing hydraulic disk controls into the fold is exciting and certainly makes it one of the most attractive workhorse Shimano groups available. Tiagra is certainly workable, but it is falling behind as Shimano advances what it considers to be its mid to high level groups.

I rode a few cycles with Tiagra groups when originally shopping. Under light to medium loads, they did shift fine. Where I found the difference to be was under heavy load. I could hit the cranks with everything I have and the 105 group would shift without error. Also, the 105 felt smoother in that instance, most likely because of finer adjustment interval. Just my take away. In the end, it only matters that what you buy does what you want. Each of us finds what works best and the awesome amount of choice makes it a fun time to be in the market.