I tell you I waffled with ZBrush and even sold it for a week, but I came back. 

Mudbox 1 is just extremely clunky and outdated compared to ZBrush 3. I sure hope the guys at Skymatter get their act together. I contacted them and they said they cannot release any information about Mudbox 2 for at least 3 months. Maybe with the Monopolistic Autodesk behind them, things will change - and get A LOT more expensive ;)

In Zbrush 3, I can spin and zoom around a 5 million poly figure without any slow down. With Mudbox 1, when I get to about 500,000 it grinds to a halt, chugging and puffing.

I have a QuadCore with 4 GB of RAM.

I also find that the stamping/stenciling tools in ZBrush 3 are far superior to what is currently in Mudbox 1.

Anyways, I wandered from the flock and am now back. ZBrush is superior (and cheaper) at this time, no question.

Just my 2 cents (from alot of painful learning).

Cheers!