I have been looking at the website of the providers above and whilst the process, material etc looks the same, the one that sticks out (and this is purely an observation) is ProPilot due to the so called "Advanced Learning Techniques".. The practical application of the theory studied in relation to various Airbus and Boeing Simulators they have on site.
This may just be a "gimmick" though and as a DL I am not sure it will be available to anyone outside of the full time residential course.
Taken from the ProPilot Website:
Modern airliner systems are now so complex that it has become difficult to understand them without hands-on experience of the hardware. Consequently ProPilot has invested heavily in advanced hardware learning aids for the benefit of our students. These include:
An A320 fixed base systems trainer
A Boeing 737 autoflight concepts demonstrator
An Airbus MCDU trainer
A flight planning station
A laptop electronic flight bag
All ProPilot students will have the opportunity to experience this hardware during classroom tuition; it’s invaluable for bringing theory to life and making it easier to understand what you are learning. KSA ATPL students will get more time on these devices with competency based exercises and assessments which deliver in depth training on the IATA competencies of knowledge, skill and attitude.
Currently ProPilot is the only specialist ground training organisation to offer on-site access to such advanced learning aids. We are also the only specialist ground training organisation to integrate practical hardware experience with classroom theory lessons.
Not much difference in cost between all providers. Will put a call in and make an informed decision.