I'd get the cannondale, I've had my CAAD for 10 years now and upragded every component apart from the forks! I just don't feel the love for Giant for some reason even if in reality both are just faceless mega-corps churing out equaly great bikes. A bit like a Honda NSX Vs a Dodge Viper, the Asian one just looks a bit too cold and clinical for my likeing.
The spec on the Giant is really quite a lot better though...
Both groupsets are now 'obsolete' 10speed, so if spec bothers you then I'd be more worried about being on n-1 gear than the groupset itself, especialy as the newer groupsets are supposed to be much nicer shifters than the internaly cabled 10s groupsets.
Road bikes don't seem to be that much cheaper as complete bikes than their parts compared to MTB's, you can get a CAAD10 frame only (which by all accounts is nearly identical to ride as the base level supersix) for £800, 5800 groupset is already down to £330 on pre order at CRC, 6800 wheels for £225 at merlin. Ok that's the budget blown before you get finishing kit or tyres, but ones consumable, the others personal preferance, and neither of you're options have anything but cheep in-house alloy kit that could be got from planet X for <£10 a piece (2xtyres, stem, saddle, post, bars ~£60) or splash out on nice stuff that you find comfortable.