The E170 is quite comfortable, but for me it isnt the question about 3 seats versus 2, it all has to do with seat pitch. We operate with between a 32" to 34" seat pitch, as a 6 ft person i have passengered 10 hours (3 flights) in 1 day. It was quite enjoyable.
Not sure if i could have enjoyed the same journey in a Flybe aircraft.
Mutt