Since you already have a Windows desktop, why not get a Powerbook for your laptop? That way, you'll have the best of both worlds. Given the systems you listed, the Powerbook will definitely not perform as well as the Alienware. However, speaking as a Mac user myself, there are intangible benefits to owning a Mac. To avoid a potential flame war (not that I'm expecting one, but it usually happens in topics like this), I won't go into that right now.

Bottom line is, if you want a laptop that was designed to perform as a laptop and not a small desktop, go with the Powerbook. If all you're looking for is sheer power in a portable package, go with the Alienware.

My two cents.