If the load order for Page Load rule is not after the Analytics tool, then Page Load Rule value will always override the value set in Analytics tool.
For example if you set eVar1 with value "Page Load" in Page Load rule with Load Order "Page Bottom" and in Analytics Tool's Global Variables you set eVar1 to "Global", the value passed in the hit will be "Page Load"
For example if I setup Evar1 in the global variable and then in page load rule, I set it to something else. I assume the order should be global variable -> each rule. So in the case, the global variable should be overwrite by the page load rule value. Correct?
I believe you are comparing the usage of Global Variables in Analytics tool and setting variables in a Global Page Load rule. If so, there is not much difference in these as both will be set with all the pages.
The difference (choice) with these approach is how you want to organize this in web property. In case you want to have a granular configuration, use of Page Load rule will be preferred.