Personally I would go for UA as service in Y is decent, in addition to having economy plus (unless you're a *S or have a Y, B, or M fare you're probably going to have to pay $79 at checkin for it). IMO economy plus is definitely worth the money (it makes a long flight in economy much more tolerable). If you flew CO you'd have to transit in EWR which is disliked by almost everyone. I haven't heard particularly bad things about either CO or UA in longhaul economy (I haven't flown CO in years so I wouldn't know).