If you're talking about tubing, I like Tange 2, almost as light as Tange 1.
Then again, I know pretty much zilch about 531. Some of my friends like it.

I've not seen Tange 2 frames in aggressive geometry, so I can't really evaluate them, but the frame in 58cm is only 2 ounces heavier than the Tange 1. Often, the fork is different.