SkyNetRising :
Aluminium casing would be better. It helps with heat dissipation.
Plastic is really cheap option.
If I made an aluminum cap for my head that would also help with heat dissipation. I don't need it however, nor do these types of SSD's.
More info: http://www.thessdreview.com/our-reviews/adata-premier-sp550-ssd-review-240gb/
"Once disassembled we can see that they did not included a thermal pad to help distribute heat into the shell off the controller. During our testing runs the controller reached up to 63C, which isn’t too bad considering the drive is fully stressed."
So there you have it. Adding metal, and a thermal pad increases cost, but won't help performance. Longevity will be generally based on the amount of data written.