Need advice....

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I have some insight on "spot heating". I have a couple of lathes, a drill press, and misc. other metal working tools in my shop. I used to use a kerosene heater to warm it while I was out there working in the winter. One of the issues with "spot heating", especially in the fall and early spring, is condensation. Warming things up for short periods of time was causing so much surface rust on my expensive tools I spent more time cleaning them then I did using them. If you plan to have any kind of machine tools or expensive equipment in the shop I highly recommend forced air heat, either NG or propane.
The other issue is cold tools. I find it much nicer to go out to a shop that is already warm. I keep my thermostat set at 51 or 52 degrees. That is warm enough to go out and putter for a few hours after work at night and it doesn't break the bank for heating costs. If I know I am going to be out there all day I set it up to about 60. That's plenty warm for doing most things on a cold winter day.
Full time heat has many advantages over spot heating. No condensation, the tools are warm, the room is warm, and it is MUCH easier to go out there to work when everything is warm.
Mark