We would be fitting the Galaxy system.
Nothing against the Scantronic Ion gear just, not used it in over 4 years.
As RFS tho they so sound cheap, where abouts in the UK are you?
Were the 3 quotes from SSAIB or NSI approved companies?
I'd always recomend proper Monitering rather than a dialing ,last thing you want is for it not to work when it needs to, in my opinion that's more likely to happen with a dialer over pstn than with monitored communicator.
Galaxy wireless is good but I'd go for the keypad rather than relying on the fob.