I would recommend something with more of a power range if it were me.
Unless this rig is just for big game hunting, then the 3-9's are good to go.
I assume since your choosing a .300 WSM, your wanting to take advantage of the extra "umph" it will give you to shoot long range.
I think the ideal power range for most tactical shooting is around 3-17x MAX.
The perfect power range is probably more like 4-14 for me and my shooting.
Just one guys opinion mind you.
I have found that more power than 17x will usually result in a loosing fist fight with mirage and a narrower field of view.
17x power or less on a hot day can be challenging enough... :evil: