halcyon :
What do I mean? Well, check the gauge of the aluminum of the Mac Pro chassis. Check the layout of the internals of a Mac Pro or the design of an iMac. The materials used in the MacBook Pro line has a good amount of aluminum and GLASS. ...might not be true but those laptops may hold up better than the plastics that you typically find. It is good to see that some manufacturers are starting to really employ magnesium and other allows in their laptop's chassis. You pay for this build design, but to many its worth it. ...and to many it is not..
mac pro chassis is interesting, though its tough to get your hands on one for a decent price, though i have seen 2 people buy them then use them for their gaming computer chassis, as for the laptop side, i have been getting PC laptops for years and never had a problem with the plastic on them or the build quality, though i tend to find the hardware is outdated in a few years needing a new laptop by then, hoping this current laptop lasts the 3 years as far as performance goes, hence i wouldn't be able to get a mac, buying a new one every 2-3 years would hurt the wallet a lot
from someone who builds their own computers, the macs aren't worth, the iMacs a pretty terrible for price, especially the newest line which uses laptop gpu's instead of desktop ones, then the Mac Pro's are grossly overpriced for the hardware, though if you want OSX you usually are pretty stuck as its either pay for the hardware or hackintosh
as for gauge of aluminum, i prefer decent gauge steel cases first, then aluminum, the current case i have for my htpc/file server and my gaming computer is the Cooler Master Centurion 590, great case for the price when i bought them ($40 a piece)