XF 2.1 What to extend?

[Robert9]

There is an addon from XON

he overwrites a mthod with this class
<extension from_class="XF\Repository\Post" to_class="SV\ReversePostOrder\XF\Repository\Post" execute_order="10" active="1"/>


now i want to overwrite his class

Should i do this (it is functional with execute_order=9)
<extension from_class="XF\Repository\Post" to_class="Robert9\ReversePostSort\XF\Repository\Post" execute_order="9" active="1"/>

or should i overwrite his class with (not functional now)
<extension from_class="SV\ReversePostOrder\XF\Repository\Post" to_class="Robert9\ReversePostSort\XF\Repository\Post" execute_order="10" active="1"/>