Short answer is the mini split.

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Long answer is it depends on a number of variables like temp and humidity, but heat pumps generally cheaper to run than a gas boiler down to low single digits. It's cheaper than oil even lower than that. So heat pumps are cheaper to run for most of the temperature range that we get here in Massachusetts. There will be a few times during the year where gas or maybe even oil will be cheaper, but those days will be uncommon.