Giant like to standardize frames and then adjust fit out to meet price. Aside from the crankset, which does not look great, I admit, there is nothing wrong with 10 speed Tiagra. The Defy is a great bike, regardless of flavour and I commend it to just about everyone. The TCR is a little racier (steeper head angle by half a degree and a slightly longer yop tube)n but my TCR and defy are set up to be the same.
For a mountain biker, I'd go Defy wth compact chainset. If you can stretch to 105 then it's worthwhile, but Tiagra is just fine.
If you want to try something from another brand; Cannondale Synapse or for race, an alloy CAAD8 race bike. The alloy CAAD8 will get you 105 groupset.
Disclaimer: I own a Defy advanced SL with ISP, and alloy TCR and a CAAD8 for Son1. All are great bikes.