Structural isomerism in iso-butyl and tertiary butyl alcohol
I think the best way to describe this isomerism is constitutional isomerism because it takes into consideration of both position of functional group and structure at the same time. 


  Constitutional isomers may have different carbon skeletons (as in
  isobutane and butane), different locations of a functional group along
  the chain (as in isobutyl alcohol and tertbutyl alcohol), or different
  functional groups (as in ethanol and dimethyl ether)


Positional isomers have the same carbon skeleton and same functional groups but differ in location of such functional groups (carbon skeleton remains unchanged). On the other hand, structural isomers the atoms are in completely different order , they arise because of the possibility of branching

I think the reason why these isomers are considered both structural and positional is that they both have the longest chain of max 3 carbons (hence same carbon skeleton), at the same time different location of -OH group  (a different chain - structural)



Hope this helps

References

Organic chemistry Mc Murry

Chemguide: Structural isomerism