Of the two XIV (inverse VIX) is more correlated to stocks. Like a massively leveraged (typically around 5x) form. If you look at where the Wilshire blipped up or down a little on your chart, you'll see that XIV tended to be a amplified version of those moves.
Whilst VIXY isn't leveraged, it behaves as though it is, around 5x times or so. And has a tendency to be more like a 5x leveraged short stock. That said, when stocks dive quickly so the VIX will rise quickly, such that it is a short term hedge (similar perhaps if you'd held a 5x short stock position at a time when stocks were diving). Of the two, XIV (inverse VIX) is more like long stock, better for buy and hold, but a massively leveraged case - 20% in that 80% cash is more like a 1x stock type exposure.
