So the arquebus is cheaper, but the caliver is so much better. Sniper just isn't that great, especially compared to 3-shot plasma!
And why are you taking Rangers at all? The Vanguard gun is a lot better, especially if you're good at rolling 6s
Rad guns can actually hurt a Wraithknight, whereas the Ranger things can't. (And yes, even at 750 you might see a WK.)
I'd also put your Warlord in the 10-man unit, for more ablative wounds.
Although the 10 man will draw more fire, and sitting a 5 man in the backfield to babysit makes more sense than sitting the 10 man out...hmmmmm.