I've addressed piercing orb vs seeker already, but I should say it certainly isn't wrong to choose piercing orb. It is a great signature and if it matches your playstyle well, I say go for it. I prefer seekers for all the reasons I mentioned.
Snowbound vs stark winter though, I'm pretty set on. Snowbound is the better rune. There are just too many times where I find myself needing to cast many blizzards - mostly when kiting a faster elite pack. Such packs aren't all that common on siegebreaker, but they're common enough - and they're the most troublesome kind, and so the ones you want to keep in mind when choosing your spec.
Kiting a fast melee elite pack (say fallen) I frequently drain my AP bulb to empty casting snowbound blizzards, and use mirror images to make up the difference. Using stark winter they would be out of blizzard more than they'd be in it. The purpose of blizzard in the build is that it is a spammable snare. If it isn't spammable (because it is too expensive) then it isn't serving its purpose that well.