The surface rust on the GSO scopes was with the earlier models.
The white tubed models don't suffer from that any more.
Which brand? Both are very good, with the usual advantage/disadvantage that all scopes have.
To prevent the base from eventually absorbing water, I would recommend that you apply (along the edges) a thin layer of super glue.
This will keep your mount in good condition for years to come.
I have members with both scopes; flip a coin
