We've just booked our third Eurocamp holiday for 2009. I would recommend them to anyone, and would definitely recommend booking early for the discounts.
We asked their staff what the difference was between the two, and their answer was that Keycamp is the brand more heavily promoted through travel agents, whereas Eurocamp targets independent bookers more. However, both can be booked either independently or through agents. Hmmm, so not confusing then.
From observation, Keycamp seems to be be the only brand promoted in Ireland, and most of their customers seemed to be Irish. Eurocamp seemed to have loads of Dutch and German customers. This may be because we went during the early part of July when most of the UK (apart from Scotland) were still at school. Tents are v similar, but their static spec seems to be quite different.
We price it out each year, but as each company presents is prices slightly differently, and because the online price is rarely same as what they quote over the phone (as per other posters) it's hard to be sure. We've stuck with Eurocamp as we now get the full 5% loyalty bonus.
I would thoroughly agree with the poster who observed that Canvas reps seem to be more -err- mature, and more integrated into their host country. For example, at La Garangeoire (a popular Mumsnet Vendee hangout) the Canvas rep is in his fifties, and normally resident in France. The Eurocamp couriers, although perfectly willing, were all students who weren't very good at thinking up solutions. If you suggested a solution to them, they would do it, but they didn't come up with the ideas themselves. And as for their French... After two months on site, they STILL didn't even greet the campsite staff in their own language. I'm a French teacher, I would not be confident that they would be able to help in a real emergency if I was run over by a bus and DH needed help with the language!