Answer
$NiCO_3$ has a higher solubility.
Work Step by Step
1. Find the molar solubility of $Ni(OH)_2$:
$Kps = [Ni^{+2}] \times [OH^-]^2$
$Kps = x \times (2x)^2$
$Kps = 4x^3$
$6 \times 10^{-16} = 4x^3$
$1.5 \times 10^{-16} = x^3$
$x = 5.31 \times 10^{-6}M$
2. Find the molar solubility of $NiCO_3$:
$Kps = [Ni^{+2}] \times [CO_3^{-2}]$
$Kps = x^2$
$1.3 \times 10^{-7} = x^2$
$x = 3.61 \times 10^{-4}M$
The Kps values are given in the book.