The green line represents the path that a long XIV+VXX portfolio have followed since its inception. To make a realistic estimation of the daily relative loss, I have first filtered out the outliers. The red line shows an estimate of the daily cost of XIV relative to VXX.
Conclusion: playing short volatility using XIV will cost you about -0.028 % per day, relative to a short VXX position (borrow cost not included).
P.S I'm not sure there is money to make by shorting both ETNs, daily rebalancing cost will most probably eat away all potential profits.
Update 22/07/2012: here is a chart of an estimated price of XIV by taking a direct daily reverse of VXX. All prices are normalized (starting at 100).