Roxy - as you will not be able to register any offspring from a Gotland x Shetland mating it really doesn't matter if you get a registered tup or not. I would go by the fleece quality and avoid white, and you could end up with lovely fleece animals.
Sorry Shankend, not trying to but into your thread... unfortunately I am not breeding my Shetlands this year due to loss of grazing..., but would have been interested otherwise.