Balance on a Thumb does seem to vary and the older models do seem to fair better than the new ones. Pretty much zero on my '89.
Quite different basses, Streamers and Thumbs. Very different tone, particularly the 5 string versions I assume you will be looking at, different neck widths as standard as well. Feel very different on strap, not just balance issues but the first position is further away from the body on a thumb, will be more noticeable the higher up you have it on the strap.
For me, Thumbs are fantastic instruments and have always preferred them to Streamers. Currently have one of each though...