Allow me to offer a few pointers. I've got both devices. Desire for myself and the Legend for my wife.
Performance-wise, there's no doubt the Desire edges out the Legend. But the Legend is not 40% slower as the CPU clockspeeds may suggest. You'll encounter the occasional hiccup with the Legend when compared to the Desire, but you probably won't notice it unless you have both devices like I do. I would call this round a draw.
Aesthetically, it's very subjective. When you pull your Legend out, people will be drawn to the aluminum unibody enclosure. When you pull your Desire out, people will be drawn to the large 3.7" screen. I think the Legend looks nicer, but the Desire is in no way "geeky". It looks better in real life than on photos/videos. I'd describe it as more sophisticated looking while the Legend is more sexy.
Surely, you've read/seen reviews of both. See here for my review on the Desire
and here for my review on the Legend
. I tried to only put down things that were not mentioned or covered extensively by others. And I mentioned a couple of differences that may or may not matter to you.
Personally, I think it comes down to your screen/device size preference.