I don't think its worth it to downgrade to another chiller. First, you lose quite a lot of money when u sell this chiller second hand and then to buy a new one. The amount you lose can easily cover quite a few months or maybe more than a year of electricity bills on chiller only.
Secondly, lower wattage of the same model also means lower power. The chiller has to run longer to cool your tank down. In the end, not much savings from electricity bill. If your new chiller can't cope in the coming hot spell, the electricity bill might even be higher instead. You can try explaining this to your wife... maybe no need to change