We moved from Servo-is to Hamilton G5s a few years ago, for Paeds. One deciding factor was the Heliox feature, which is easy to use than on other vents.
Very happy with them, they work well from small babies to adult patients. Users have no problems, no major faults have shown up, good company support. And you can get cheap disposable expiratory valves, unlike the £800 autoclaveable block in the Servo-i.