How can I tell which road bike components/groupsets are better?
Typically the majority of their yearly R&amp;D is spent on their top of the line group, then they will trickle down it's technology to the next lower group, and continue that way, so usually, this years tiagra is close to last years 105, etc.   The major thing to look for is # of speeds in a group, weight, and finish.  

I believe the 2010 Sora(3400) set is a 9 speed, where the 2009 tiagra is a 9 speed - the major difference is the thumb button of Sora(3400) versus the finger paddle of Tiagra(4500), a substantial difference.   

the quality goes 2200 &lt; Sora &lt; Tiagra &lt; 105 &lt; ultegra &lt; dura Ace