I own ASUS P6X58D Premium myself and was able to get i7 920 to 4Ghz. Likewise, many other people did it as well on their 920's. I'm happy with the board and I'm sure it will serve you well. It's advantages over the Extreme is that it has USB3.0 and SATA 6Gb/s ports.
Now, I have looked through the manual of R2E and it does offer you extra options for super fine tuning and tweaking overclocks. Not to say that the P6X58D does not have any, it's just R2E is the "overclocker's class". I was considering that board as well. I think, if you want to squeeze every little drop of juice out of your cpu, dedicate time to doing it right for additional 200-300Mhz while spending more $$ on the board and giving up the little extras, then go for it. But, there is also no guarantee that you get lucky with your cpu. Some samples just don't OC well, no matter what board you stick them in.
In any case, either board will be a good purchase. I'm happy with mine and assume I wouldn't be disappointed with the other one.