They aren't top of the range nor bottom of the range, you need to look at the specs and decide which is the best for you.
The HDD is a significant part of the cost.
As for support that is upto the manufacturer and they could pull it tomorrow or support for 20 years there is no way I can know that for sure.
NVRs I put in 12 years ago are still working fine don't believe they are updating anymore, but if it isn't broke it doesn't need fixing,
However lets say The USA or UK fall out with China and place restrictions then things may not work, there was a period where you had to use the hik store to get the latest app version for example.
In our industry seems to be a rush to market and sort the issues out later, so you may have issues if you use the very latest tech, not had to do much with the M series, however there was issue with how to add certain doorbells/ intercom systems and had to be done a certain way depending on firmware versions.
Get people buying grey imports as they are cheaper but they cant always be updated.
I’m not desperate to spend a fortune a top of the range NVR I’d just like one suitable for my requirements so will gladly accept your expert guidance on which one is best for my situation.
On the information supplied against the specs I have seen for those products I would look at those 2 options and make the decision.
I can certainly see the Amazon Hikstore (supplied by bits and bobs) works out cheaper but no idea who they are or what they are actually selling and what level of support you would get but certainly you can get the stuff cheaper that appears to be the same stuff.