Reading this forum enough, and reading Burgerman's website enough, I think you'll find that the question is not as important as it sounds right now.
The main problem with 99% of powerchairs is that all of the control inputs are delayed, they lack power, and the weight is not on the drive wheels. I have tested a Quantum R4000, and it had all of these problems in spades. If the chair is programmed properly and your seat position is more or less on top of the drive wheels, then the different traction control gimmicks are much less necessary.
Now as far as the gearless brushless or the geared Invacare motors, that's a harder question. I have seen some of the recent Invacare geared motors, and it is true that they go less than 6 mph. The motors look superficially the same, and I would be surprised if there is anything different about them than a different number of windings to produce the lower speed. If you want to go faster in an Invacare, then the GB is the only way to go these days. That's what my wife is doing, and we're paying the extra for it.
I've heard that the GB motors are extremely reliable, except when they're not. That sounds funny, but what I mean is that they sound like they have a tendency to work great if they're not broken, but if anything goes wrong then they immediately stop working without warning. With traditional geared brushed motors, you will start to notice noise and a lack of power or range, but the motors will hang in there for a while as they degrade. The GB motors include various electronic sensors and memory that need to be in good working order for the chair to drive at all. As long as they're within the expected bounds, you're good. If one little wire comes loose inside, or if a sensor starts acting up, or various other subtle problems, the chair could stop dead. That's just inherent to how they've built the motors. If they didn't have so much safety checking, they'd probably be more reliable but might give a rougher ride or be more likely to continue operating until something actually burnt up. The designers deemed it to be safer to have the chair stop dead instead of potentially igniting, so it is what it is.
My problem with the Quantum (and other Pride gear) is that they seem so very guarded against anybody using it possibly understanding it, or working on it for him or herself. They don't make it easy at all to get your hands on a good programmer to take care of all the quirks that need to be ironed out from the user programmable settings. I've seen one person who had a good wheelchair technician working with him, and his Q6000z chair drove fine. Someone else I know had to have hers sent back to the factory (permanently) because the techs couldn't figure out how to set it up so that it wasn't a nightmare.