personally, I would go for the alu.
I have had both and the brightness of the screen at about 40% is still a tad brighter than the old ones. Also, the build quality of mine is incredible, it looks gorgeous compared to anything else out there, including the old one. It feels incredibly sturdy because of the alu compared the the plastic. Also I have the back lit model, and it helps when in lectures.
However, the killer feature for me is the trackpad, it is just phenomenal, its size, texture, clicking, multitouch, it makes things so much more efficient (like moving forward and back through webpages, right clicking, rotating, etc).
Whether its worth the extra cash, I think so, many people think otherwise, and thats cool, we all have our own views.