I've just gone through that same debate. I suspect that they will be an improvement since they have XStat instead of the previous panel design. How much of an improvement remains to be seen. Since I was looking for something in the immediate future, I went with the Source since ML says that the ElectroMotion will ship in March 2011.
By the way, as demonstrated by this debate, preannouncing an "improved" product seven months in advance is known as killing your own market. It freezes sales that are about to happen while everyone waits for the next great thing, forcing dealers to discount heavily to dump existing stock.
The press made a big deal about other phones outselling iPhones back in the first/second quarter of this year. Yeah, but everybody and their mothers knew that Apple was going to release the next generation iPhone, so not many people were buying the older model, except under significant discounting. In that case, Apple didn't even make any formal announcements - the market froze based on rumors and previous Apple behavior. In this case, ML themselves officially announced the product AND the expected release date.
