The main difference between the two is 9 speed vs 10 speed. The Tiagra shifter has the old shape as the 105 shifter has the new shape. Some like the old shape better and some like the new. The Sport has 11-32 vs the Comp's 11-28. For loaded touring, I like have a 32 cog as a bailout gear on the climb. 9 speed components tend to be a little cheaper if you are to replace them and the chain tends to last a little longer than 10 speed because 9 speed chain is not as narrow. My gf has Tiagra on her Cannondale and has no problem at all for the past 3000 miles.