I seriously doubt it would be worth using as a space heater, the heating efficiency is going to be super low and you can't control the temperature. The bitcoin that it will generate is maybe $5 a month. So yeah I would vote to just buy a real heater.
Depends on how you look at it. First off, heat is heat. Yes it's what only 350W or so but that still equates to a normal small heater running at the same power. Does it 'pay for itself' or earn more than the electric bill for it running? For any Ant model lower than an under-volted/clocked s7, hell no.
However if you *are* going to run a space heater anyway during cold weather, what a miner earns does offset the electric bill for the heat. So win-win there. One of my so far good R4's is running in my office just for that reason (and earns more than it eats)