MacOS has better hardware security and you will actually get firmware updates. Software compatibility is also far better on MacOS as compared to Secureblue. NymVPN for example does not have an rpm package currently, which means you can't layer it on Secureblue.
I would recommend you read these and make your own descision:
This has also a lot of good information, but it's already kind of outdated. The Latitute laptops got replaced with "Dell", "Dell pro", etc. And most of them (vpro models) don't meet HSI-4.
Vpro Thinkpads meet HSI-4 more often, but they cost so much that it makes Macbook pricing look reasonable.
I'm currently waiting for the a18 budget macbook, which if it's priced as rumored, would be the cheapest secure device I can afford.