I tested both browsers with Sunspider and here were the results (lower number is faster):
Beta 12 - 7392.0 ms
4.0s - 7724.3 ms
Yeah, I know, but I'm on a Sawtooth. Cut me some slack.
Then I went over to Dromaeo, and the results were (higher number is faster):
Beta 12 - 18.89 runs/sec.
4.0s - 16.78 runs/sec.
So should you use the beta instead of the final? Well, the final had security fixes, but I have a need for speed so I'll stick with the beta.
Danger is my middle name.
Addendum: Lest you think the final release is some kind of slouch, it still beats the pants off Firefox 3.6, whose Sunspider score was 18786.2 ms, a full two and a half times slower. So if you're using Firefox 3.6 and not TenFourFox, then you probably haven't read this far yet 'cause your renderer is so slooooooooow.