If I ever knew, I've forgotten. When I looked at the options it was pretty clear that there are essentially two types of Arduino. There is the Uno, Mega, and those of that ilk in the mainstream, and there is the rest in the peripheral niches. On looking at the options, the niches very peripheral and the mainstreamis unsurprisingly where you find the bang for the buck. Most pointedly, you would need to see something pretty convincing to pass up the Uno you already have if that can be persuaded to do what you want at no cost.