First question is what sort of riding do you want to do? General training in foul weather? Ride some sportives? Try racing? Road bikes come in almost as many flavours as mtbs, and each has their merits.
Of the two, the triban is a better all rounder, with room for mudguards. The Merlin is a “full-on” race bike with classic geometry, both are however excellent value.
The Giant Defy# gets a LOT of love here because it is an accomplished “first” road bike that is a perfect compromise between more comfortable, endurance riding, and full on race bike – indeed a Defy won the prestigious Paris Roubaix classic last year. The bike linked to at Pauls Cycles has the same race-winning geometry.
For your budget, I’d pick the Merlin for me, but the Defy is a better choice for a first bike.
I don’t have a road bike btw. I have five 😳
#Think of it as the road equivalent of a Giant Anthem. Accomplished at most things and more bike than most people will need.