The tee's are cotton so they'll breathe reasonably well, obviously the fitted ones will absorb more sweat than a loose fitting one.
I find their cotton is a lot better than most mainstream brands, a much more premium product. You can feel it in the weight/fabric but I rate it for sure
If you sweat a lot (or are on the epic), get the gym tees or singlets. The BFG's and long sleeves will be too hot. I like them instead of a jumper on those in between (not too hot or cold) days but probably couldn't train a full session in them.