Really not sure what to do.
I’d really like a nice summer bike, I have a budget of £1500 for this.
I would be buying used, I did plan on a TCR Advanced preferably the disc version with Ultegra groupset. However I’m now not sure if it would be a bit uncomfortable for me, I’m not in a club nor do I do races etc. So maybe a Defy would give me a little more comfort and still be quick and light too.
Don't listen to the comments about one bike being really fast compared to another, the difference is infinitesimally small between a race geometry and tour bike for the vast majority of normal people-excluding professional riders.
The only thing that really matters first, is comfort. features and reliability are secondary to comfort.
If a bike makes your neck, back, or wrists hurt, because its too aggressive in position is a pointless bike to you.
Oh its always the engine on top of any bike which makes it fast or slow.
Just watch GCN on Youtube where they compare cheap bikes versus super expensive bikes, its a handful of seconds with literally thousands difference in price.
Hope that helps