I’ve got a Propel, really nice bike but it is an aggressive position and ride and although it’s perfectly good for general riding it is more at home racing.
The TCR might be a better option if you are only racing occaisionally but it is a very different geometry, should be a lot more tolerable depending on how flexible you are.
I’ve had 2 TCR’s, one was about 10 years ago, I totally loved it, at first I put proper TT bars on and used it with a disc wheel and tri-spoke, and did my first sub 60 25MTT on it, still not gone much faster on a proper TT bike.
Then changed it to be a road bike but with clip-ons, did ADH tri on it and lots of miles with no problems. It then started aging.
I got another about 2 years ago and struggled to get on with it, I could feel my left leg having problems, but I think it was more likely to be me than the bike
Anyway, you can’t go wrong with a Giant really. If you aren’t racing that much I’d probably go with the TCR, if you do or can get away with a firmer ride and aggressive setup go with the Propel. Does he have any old ones you could test?
Finally, can you use cyclescheme from work, it will save you a fair bit of tax.
My road bike is a 2013 TCR advanced and it’s a good bike. It’s comfy enough to do some good mileage on and likes hills, I had clip on tri bars and could do some reasonable times on it. The propel is supposed to be a stiffer ride and racier position and I’m not sure I’d want one for riding all the time. Then again, I’ve now got a TT bike so it doesn’t need to do double duties.
If I can ever bring myself to pay new bike prices (sorry, how much for an alu frame with Tiagra now???), I’ll be replacing the TCR with something more do it all/endurance focused like a Ribble CGR and just use it for training rides.