Though for web browsing both presumably fall solidly under "good enough".
If I was signing up for Tech Support for Relatives, though, I'd _definitely_ go with the Mac; you don't want to be getting phonecalls about Windows 12 or whatever in 2027.
M1 was faster than a 5600g, I think, but you can get a 5-series micro PC for cheaper than even the cheapest Mac Mini. For 'browsing' operations, I suspect it wouldn't matter much. There may be some advantage to using macOS vs. Windows, if this is aiding a family member... since I assume the IT support burden would be lower.
Most JavaScript isn’t dependent on that specific a feature for performance, however. In most cases what matters more is that they have great memory performance and handle branch-heavy code well.
Mozilla also implemented it. I believe Chrome/v8 have as well, but I know there was some work there to avoid accidentally enabling it on older ARM chips which don't have support for it.