Personally, you could classify me as one of those CB'er people who is the 11th level of Purgatory and recognizes that paying the Prosper loan helps offset the credit score, each month of perfect payment history is one little teaspoon of help to fix the damage.
That said, I would put the Prosper loan below the house payment, below the water/gas/heat, but above the luxury utilities (satellite/cable, phone, etc). Since my revolving debt is zero I'm not having hunger pangs over it. I've already re-sized my life around voluntary carlessness (having a car where I live is a major pain).
But that's me.
If I was one of those countless suburbanites out there who think nothing of running up debt, then I'd be paying the Prosper loan up until the point where my careless spending finally catches up and you hit that day where you realize every card in your wallet comes up DECLINED, or you run a few days a week searching the car and the house for loose change so you can rub enough nickles together to put gas in the tank to get to work.
And then to make matters worse--just stop paying the Prosper loan entirely so that frees up some money to continue spending money on crap rather than spending the 3-4 hours it takes to come up with a quick budget and plan the spending.