Hi Nick,
The 5TT tissue types are really just rule masks for tracking (eg termination, exclusion etc.). This article, specifically Fig 2 lays down the rules for the first 4 tissue types. Unfortunately it does not give any details about the 5th tissue type.
In short, from my understanding, the 5th tissue type doesn’t put any constraints on tracking. I.e. it allows streamlines to enter it, terminate, exit etc…
To directly answer your questions:
what would be the consequences of including the WMHs as either the 3rd (WM)
In this case the lesions will be treated as “white matter” i.e. streamlines will propagate through the lesions but if a streamline terminates in a lesion its will be excluded.
what would be the consequences of including the WMHs as ... 5th (other)
In this case (as mentioned above), a streamline can enter it, terminate, exit etc… This is most likely the behaviour that you are looking for.