I’m biased as I just bought a Defy 2 for winter, my conclusions were
Tiagra on the Defy, Sora on the Triban. Main difference being the downshifting on the sora being a button up on the hood rather than a lever you can reach when in the drops. I think there is also a difference in being able to shift down multiple steps on Tiagra.
I’m not sure, but the triban doesn’t look to have points on the back for fitting mudguards ? Defy does, and there are giant specific guards that fit without issue.
From others comments I gather the saddle on Triban will need changing right away, more than offsets buying some pedals. I use M520 mtb clips. The Defy’s saddle seems fine.
The Triban’s forks are carbon, while the defy is alu .
Most reviews seem to say the Triban is a great bike for £500, and the Defy is a great bike for £800, if you can find your size for £200 quid less then you should be doing pretty well, I paid £100 more than that.