Electric is actually cheaper here, in the heart of hydroelectric, than any fuel I've compared it to. But I know it's more expensive for most people.
One consideration with ground heating systems is that you can lose half your heat right into the ground. Proper, deep insulation and radiant reflectors are vital for a ground contact system.
I heat the water in the big koi tanks in my large greenhouse, and with the all the falling water they put a fair bit of heat (and moisture) into the air. Electric air (not radiant) heaters kick on when needed. And when it get particularly cold I have propane as my emergency backup.
But I think every situation calls for a different mix, and it can pay to do your homework in calculating BTU loss in your structure (easily done by measuring steady state temperature before and after adding a known BTU value heat source on a night when temperatures are fairly constant), and calculating BTU/lb of fuel costs so that you're comparing fuel costs properly.
I do have venting when I run gas, and I keep CO alarms in there, just in case.
Smaller structures require simpler systems, but can go wrong faster, too. I always having monitoring on any greenhouse containing material I care about.
