I'm debating the same thing! Except I only have 1500 pts of guard, and it's a blob-mech combo. It seems to me that if you have 5000 pts, you must play a bunch of apocalypse, so a Shadowsword might be a good centerpiece for you. Destroyer weapons are supposed to be amazing in apoc games.
If you play on a more standard-sized board where the deployment zones are 24" apart, I'd say a Stormsword. With the prevalence of cover saves, a S10AP1 weapon that ignores cover is invaluable. And the size of that template means death for whatever squad you pick as your target. Personally, since the max-sized game I'd play in my meta is 2500, a Stormsword is my front runner.
Oh also, not sure if you're aware of this, but the new Forgeworld FAQ states that the ordnance snap-fire rules no longer apply to superheavy vehicles. So you can move at combat speed and fire ALL weapons, including your primary, at full ballistics skill--and every weapon can fire at a separate target!
It's good to be Guard.
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