Macs are fine for normal day to day stuff: coding, web-surfing, development, etc. But per dollar they're terrible at raw throughput.
For pen testing you likely want a laptop, not a tower, since you'll be doing things on-site a lot of the time. But a laptop (any laptop) is the opposite of what you want for raw throughput due to heat dissertation issues.
So I'd likely recommend a really barebones tower with high spec (e.g. unbranded/self-built, i7, high end GPUs, etc) and a laptop (e.g. Thinkpad T series) for the user/pen testing stuff.
I would always want more computer, and even the biggest i7, SSD, 64GB ram... machine is going to take longer than you want to pull up HN if you are running intensive tasks.
But, yes, if your time is worthless utilise Linux. If time is money then something better.
Before anyone from the Apple camp chimes in, I had a similar experience with my old laptop, which was a Mac where the file system corrupted itself every time it went to sleep.
I'm not saying that Linux doesn't get ugly. It does. I've had the failed sound cards and bad printers. Hell, I'm old enough to remember screwing around with trying to get a WinModem to run. It just comes down to whether it's cheaper to waste ten hours of your time trying to get things working or to return to the 20th century for a week while support reformats your hard drive. That probably just depends on your occupation.