Hey guys, you know there is NO difference between 100% aluminum and the acrylic notebook coolers. First most notebooks have a plastic outer shell which does not conduct heat very well. Second, the only contacts the notebook will have with the cooler are the rubber feet of the notebook and the rubber/silicone stand offs of the cooler. In fact rubber (and I think silicone) are insulators. The coolers work by adding to the airflow underneath the notebook by elevating it off the surface and by increased volume of air. It does remove some heat through the plastic, but the main purpose is to provide cool air for the internal fans of the notebook. The only additional think that an aluminum notebook over an acrylic is that since the aluminum is in contact with your legs. It will act as a heatsink for you, LOL.
Having said that, base your cooler decision on cfm's of the fans, noise level, comfortness, and of course price. Also these things do make a huge and noticeable difference, especially with those people who have real graphics card in your laptop. My Dell 8500 and 9100 used to get pretty warm on the keyboard, before the cooler. Now I don't notice much of an increase on the keyboard. Also there used to be a hotspot in the upper left corner, which is gone whenever I use the cooler.