Need Advice: Split Units or AC?

I’m currently renovating a two family home in the northeast that runs on gas via one boiler.

I’m trying to figure out how to separate the upstairs heating system from the downstairs system. I don’t want to add another boiler because that’s at least $20k between the cost of the new boiler and the plumbing. The most cost effective option seems to be split units, which would still be around $10-15k for the units + the labor.

The entire house also has in-wall air conditioners. Would it be more cost-effective to buy in-wall air conditioners that provide heat and cooling instead of paying for split units? All opinions are appreciated!