In doing my research before buying our CtP fares, it really appears to be mostly a convenience/time vs. price trade-off. As you've probably seen, there is some difference between the pricing of the different carriers, but those differences are mostly due to the relative strength/weakness of the carriers' home currencies. If you're willing to put a lot of time and effort into it, I'm sure that you can save 5% to 10% (maybe a bit more) by looking for the best currency conversion rate.

Now, I'm sure there are some differences when it comes to the service/ability of the airlines call center as well. Personally, I chose to book mine through my main airline, UA, for convenience (I do not have to deal with the ICC!), even though I probably could have saved a few bucks with more searching.