America's Test Kitchen did a test of Grill Pans. I don't think I can link it, because I think you have to be a member to access the article. I link it below anyway, so you can give it a try. Meanwhile, I will copy the pertinent info:

"...it soon became apparent that aluminum was the only way to go. Why? The aluminum pans were lighter and so easier to handle, and they produced more distinct grill marks than their cast-iron counterparts. (Which makes sense, given that aluminum conducts heat more efficiently than cast iron.) What's more, it took just one ultra-sticky round of glazed salmon to make me decide that nonstick aluminum is a better choice than a traditional surface. Scrubbing the sticky, burnt glaze and salmon fat from between each and every ridge was, to put it politely, a major chore with both cast iron and stainless steel..."

So, among the aluminum grill pans, their top winner was the Simply Calphalon Nonstick Grill Pan. It is a round grill pan, but Calphalon makes a square grill pan in their Calphalon Contemporary line which probably works the same.

Simply Calphalon Grill Pan

America's Test Kitchen Review of Grill Pans

I've been doing a lot (relatively speaking) of searing lately... since we're in Orlando, FL, we can grill all year long, but choose to sear as of late. I use a SS pan and Medium High heat.

Having said that, I wonder if a non-stick grill pan makes sense. I've read here and other places that a non-stick pan should not be used with temp greater than Medium heat, otherwise the non-stick coating will emit dangerous fumes.

Per the ATK link: "(Besides, for safety's sake you should not overheat an empty nonstick)"

-Gorilla