The Mac mini used to be easy to upgrade, although the version introduced in 2014 isn't as upgradeable as the past models had been. However, it is possible to swap out the hard drive if necessary, although you can no longer easily upgrade the RAM. So be sure to buy the memory you need with this model.
The new Mac Pro is an interesting unit in that it replaces the one Mac with superb upgradability options with a radically new design. You can upgrade the RAM fairly easily, but you can no longer install PCI-Express cards into the Mac Pro and the SSD. However, the SSD isn't soldered to the board as in a MacBook Air, instead, it sits on a card above the GPU. Tantalisingly you may be able to upgrade the graphics card down the line, although the CPU will be forever fixed.
Upgradability is a mixed bag. On the one hand, there are size, weight and performance advantages to having everything soldered inside the machine. But you can easily extend the life of a Mac by updating the RAM and hard drive as prices fall. Again, the Mac mini and iMac 27-inch offer longevity.
Conclusion: Should you buy a Mac desktop or laptop?
It may sound trite but in the most cases it really does come down to 'how portable do you want your computer to be?'. If you need a machine to carry around with you then the MacBook Air and MacBook Pro offer this feature.
But if you don't need portability, you really get much more from a Mac desktop than just a bigger screen. The faster innards, and upgradability options (at least on models other than the 21.5in iMac and Mac mini) will allow you to buy a machine with more potential lifespan. And don't underestimate the productivity gains of working on a large 27in display (especially one as beautiful as the new iMac with Retina 5K display).
The only thing that lets the average desktop down is the slower hard drive, so we recommend you upgrade to flash storage, or a Fusion Drive when you purchase it.
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