New build, should I get oem or retail Windows 10?

I'd like to build a budget system for my mom. Should I get the oem windows 10 or spend the extra $ for the retail flash drive version? The plan is to have this be a long-term system, which means the oem version should be fine. However, if the motherboard dies in five years, post-warranty, then I might be in the position of having to buy a new windows license for a new, different, motherboard. Any thoughts?

The OEM version is fine and can be installed no problem from a USB drive. The major difference is support from M$. When was the last time you called M$ for support?

And I have built with OEM licenses for MANY years. I have never had to buy a new key when I had a major hardware failure. I just call the key activation hotline, and they re activate it. I have also done with successfully for W10. OEM should be fine.

If there is a failure in 5-7 years, would a suitable replacement motherboard even be available for sale anymore (for something like a haswell pentium)? Or by that time would it just be easier to build a new modern budget system, which wouldn't be able to use the old oem license that was tied to the haswell motherboard?

I've always used OEM system builders edition when building PC's for people. You can still call Microsoft for product key support since there's no way an OEM can manage the licensing on their own.