It would depend on the circumstances - if it was a last minute requirement, it'd be a case of whatever could be rustled up. GBRf don't typically have a lot of 66s in and around Scotland (and available), nor that many with RETB fitted. It'd definitely an option, but so would a 67 depending on what DB had in the area (there is currently one stationed at Craigentinny, although it is there to be the LNER T/bird).
If there was a longer-term known requirement (e.g. another 73/9 needs to go to Eastleigh for a few weeks for new wheelsets and another one is undergoing repairs, say) then it'd probably make more sense to hire a 67 for the month.
I suspect we'll see 67004 hang around a little while until the Mk5s have bedded in on the Highlander - and 73970 has made it back from Eastleigh and is back in service.