Maybe just another S9, depends. How much heat do you need and how often?
You can run 1, 2, or 3 hashboards at a time on an s19.
Discussion
I usually had it turned on from November until May, clocked to 1250w. Main purpose was to keep the pipes my cabin from not freezing during winter, the few sats I got was just fun.
An s19 can hit that wattage with only 2 hash boards enabled. Up to you if that cost difference and w/th difference works out in the long run.