Can you test ride one? My lbs(BFold of NYC) have a C17 and a C15 loaners/testers; as well as some other models/brands.
The C17 was okay for me on up to 50 miles; but started to get uncomfortable for longer rides. When they first came out;
you didn't have the cut out option.

"My main concern with my leather Swift is that everytime when it rains I have to stop and put on the rain cover, which is a hassle and often times I get dropped by the group I'm ridding with."
If there's a chance of rain; just go ahead and put the cover at the beginning of the ride. Then you wouldn't have to stop.
100 Miles(160 Km.) on a Brompton by 1nterceptor, on Flickr
BROOKS C17 LOANER FROM BFOLD by 1nterceptor, on Flickr