The stats you see take into account the affection bonuses, all the same the additional levels let them reach higher limits and if you don't want to take a power loss initially have their affection maxed before you evolve them. Improving their ability can more than make up for the unit stat loss in some cases as well St Paulia for instance changes from +3% Atk for 2 units to +5% Atk for 3 units after evolving every little bit helps and with max affection before evolving her HP and Atk both increase taking a small loss in Def that she's already regained anyway.