Deciding higher priority order between a triple bond and tertiary carbon
From the Wikipedia article on CIP rules.  


  If an atom A is double-bonded to an atom B, A is treated as being singly bonded to two atoms: B and a "ghost atom" that is a duplicate of B (has the same atomic number) but is not attached to anything except A.   


Ghost atoms are attached to resolve the triple/double bond.  

So according to CIP rules the two compounds can be resolved as 

 The pink atoms are ghost carbons   

From here we can see that ethyne has a greater preference over isobutyl as after the first point of difference we move over to the next highest order carbon.In ethyne the second carbon is connected to $\ce{C2H}$ whereas the next highest order carbon in isobutyl is connected to $\ce{H3}$.