Can't answer your question about why OAT are more expensive, all I can say is that I trained at Cabair and was very happy with the training I recd, both through the ground school and the flying, and from my experince there I would recommend them, bear in mind I was there in 1995. As far as getting that all important first job I don't think there's much difference, some airlines have preferences, but as long as your expectations aren't too high I don't think you should base your choice on which school reckons it has contacts with various airlines, because they all know each other.
Hope this helps, if it was my choice again I'd go for the cheapest, as long as it's the CAA Approved training it's the same licence at the end of it all.
Good luck.