ash34 MTE somewhat increases resistance to attacks, and also helps developers uncover bugs that might be exploitable.
If the device will run just one app, that you trust, and will be used only on a network you trust, it's not particularly easy for it to be attacked, compared to devices that run lots of apps and visit lots of web sites. It's not clear to me that the added expense would buy much.
However, the 8 has almost six years of likely support, compared to around 3 for the 6a (source). That's a big difference!