As Matt suggested, please check out the system requirements before buying a new computer. In general, a discrete GPU will always be better.
While the X-Plane team uses Mac computers to work on X-Plane, in all honesty I would recommend purchasing Windows computers if you're already in the market. This is simply because you can get a much nicer graphics card for the same amount of money you would pay for a Mac computer, and then you can even upgrade the CPU or GPU separately in a year or more if you have a little extra money to spend. Macs are a single unit and cost thousands of dollars to upgrade to new ones.