It will behave a base in water. Acetate salts undergo the following equilibrium in water:
$H_3C-CO_2^(-) + H_2O(l) rightleftharpoons H_3C-CO_2H + HO^-$
Given a $pK_a$ for acetic acid (which I think is $4.76$) you could quantify this expression. Anyway an aqueous solution of rubidium acetate will be slightly basic given the hydrolysis reaction.