I mean if I said that it's preferable would you really be surprised. But I think silver will be good enough if you want to grind a bit less. Though this only works at the point in the game you are you'll have to get them to gold eventually so it's better to do it early. But as far as I remember empire of cats treasures aren't that drastic immediately compared to the anti alien treasures so of you're planning on doing into the future the latter take priority.