Mine just didn't hold up well, when I backed over it with my car. It's hard to understand why it didn't.
OK, all kidding aside, the only drawback of aluminum I've experienced in all my MBA's & MBP's is it impedes the performance of WiFi. Yet that's no surprise to those who know their materials.
"Plastic" is too general of a term, and there are about eight popular formulas used by the various OEM's & ODM's depending on price point.
Top of the line T & W series ThinkPads use CFRP. That's Carbon Fibre Reinforced Plastic. The same general type as used in Aerospace, IndyCars, F1 etc. They are the most durable of all, and do not limit WiFi performance.
At the end of the day, both Macs & ThinkPads are the pinnacle of laptops. I know from first hand experience using both concurrently year after year.
I've had both the 2010 & 2011 MBA's & really put a lot of time on them. I highly endorse either size, it's all in what fits your needs. I will also ad, they are nearly as strong as MBP's and certainly nothing to worry about.
You absolutely cannot go wrong with a new Mac.