Tricky decision. The Carrera Subway IS a good bike; well, according to the reviews. I have a friend that has one and it's given her faultless service. I don't know that particular CB but I have a CB Cape Wrath mtb and it hasn't skipped a beat.
Got to say though that back up service is a factor here, perhaps the deciding one. Carrera is
Halfords and I hate to say it but I really wouldn't want to rely on getting decent service from them. Personally, I think I'd go for the CB. But, as Mr Paul says, see if you can give them both a try - it's you that's got to ride it day after day!