I like the Defy (I ride what is pretty much a Defy clone) and find it exceedingly comfortable. I will Always suggest the Defy as a road bike .... however ... (as others have said) ...
If you have a bike which really works for you (the Domane) why not ride it? It does everything you want and you know it does it to your satisfaction.
You could throw a $100 Craigslist bike on the trainer. If the frame fits your build even a little, set up the bike to have the same contact points as the Domane and it will feel exactly the same ... on a trainer. Weight won't matter, wheels won't matter, tires won't matter .... except for the drive train, parts quality won't matter.
I have an ancient BikesDirect bike which I used on a trainer for a while .... it is a 52, while I ride a 56 or a 58 .... but with a ridiculous stem and a really long aluminum seat post I made it fit. Feels fine even though it looks like a TdF bike (those guys also ride tiny frames with cubit-long stems.) I used my oldest, heaviest wheels and leftover cheapo 23-mm tires I would never ride on the road ..... On a trainer it makes zero difference.
Of course, I would probably get kicked off the site if I didn't advocate N+ .... so buy the defy, And buy a cheap bike for the trainer.