Normally a Spotive type bike has a taller head tube & longer wheelbase Cuch (I have found through my avid reading) & this provides the rider with a more upright position & stable bike.
Your right that you can set them both up to be racy/relaxed but, & this is only my opinion, you couldn't get a relaxed geometry bike such as the Synapse or Defy as slammed (as the kids call it) as you could the SuperSix/CAAD or TCR. It's all to do with the headtube height & trail.
Changing a stem to one with a large angle, say 17° & adding spacers etc could give a more relaxed position to any bike & a low 6° angle stem (mine) & angled down with spacers removed (bar one can get it pretty slammed) there you go, I am now down with the kids.
While its true that you cant slam the defy as much as the tcr there is only 15mm in it and while the tcr is a more responsive bike that doesn't make it unstable compared to the defy . There defy is only 20mm longer than the tcr from centre of the crank to the centre of the rear wheel .
My tcr and defy are both set up at the same slammy level . That wasn't planned its just how I have adjusted each bike to suit me and having just checked it how they have ended up .
I'm sure there is some argument for race and relaxed geometry but it seems to be a massive point raised when people are buying a bike and I don't think it is a massive detail to be bothered about . Compact or standard double would be a bigger issue for me .