Hi, most of them are very similar in performance, but as a matter of reliability and support I have been dealing with a company called Wel Medical from Aldershot. If they still have the same deal going, when you buy the Defib, they will also supply you with the Trainer version as part of the deal. They will also let you have a training defib on loan before you buy the actual defib if you would prefer that. It is always good too be able to practice with exactly the same defib that you will be using. I also have the G3 and its a very reliable machine to. There are many reliable machines around but the prices vary considerably, personaly I would reccomend the IPAD (Intelligent Public Access Defibrillator) that Wel Medical supply, it is the same size/weight etc as most defibs, and is a 123 operation.
http://www.ipad-aed.com/
Hope this helps.
p.s. if you do contact them, mention me ...Barry from Foxes Academy, you may get a better price;-)