ATP
I cannot offer any input on the citizenship requirement of ATP but I can speak to the quality of training. At a previous airline I was an instructor on the CRJ. The company would hire pilots from varying career paths including freight guys, night haulers, VFR caribbean pilots, instructors etc... the best students I ever had bar none were those who came through the program at ATP. The Director of Training at my old company would actively recruit guys who had completed the ATP program or the CAE program - both of which had an airline prep course designed for flying RJ's because these guys had the highest success rate in initial training. This is just my personal experience - I do not work for ATP or know anybody who does and have never trained there myself so take this for what it is worth. Just one guy's opinion.