From what I've heard, the VXX and VXZ are less correlated with the VIX index than the pure futures are. Hence, I would prefer trading the futures even though that means rolling the instruments at significant cost.
Why would their be cost? Don't some futures brokers auto-roll over your contract? Or is that a separate instrument called a "continuous contract"? What would be the costs associated with manually rolling it over? Two commissions?
There is not an actual "cost" (aside from two commissions) when rolling the contracts, but often a large difference in price level, eating away your profit potential. For example, if you buy one VIX future which expires at level 16,5, the next months future may at the same time be at 17,5, so you might lose a point which would not be very good if you are bullish on volatility.
Chart the $VIX spot index vs. the VXX ETF. That gives you approximative answers to all your questions. VXX includes all backwardation, contract rolls and fees to simulate slippage on contract rolls.
Then you're not trading VIX futures. You're trading a product they created and they're arbing you against VIX futures with your eyes wide shut.
What products are these? I only know of the VXX, VXZ, etc. They are expensive, but cheaper than doing the roll yourself every night if that's what you want.