Heat source for apt. buildings????

Two buildings, 3 units each. I currently pay heat. Both blds have oil furnace. One bld. has natural gas that heats water.
Oil is over $3 a gallon, and I think I use around 15 gallons a day in each bld.
I have in contracts that I have the right to change heat source, and I can have them set up their own accounts to pay. (Obviously their rent will be adjusted accordingly)
I have been bit by tenants not paying, and by the time I get them out I'm down a bunch of money, AND I've been paying their heat the entire time they are not paying me!! haha Good system they have.

If I can change heat billing over to them, at least I'm not stuck paying for their heat/oil while they play the system.

Options:
Natural gas.....I have to pay piping and for heaters. (Water is already seperated and Gas company says supply will handle the units I would need)
Propane.....Propane companies won't put billing in tenants names. Why pay to change if I still get bills??? None.
Electric......They already have to pay their own electric, w/oil being so expensive I'm not so sure it is a better deal than electric. (still researching this) I can run wires to each unit and attach heaters at a good cost.
I used to wire and did my own house, one tenant is a Master Electrician and is currently out of work. I would get a good deal with him.

Let me know your thoughts!

The different options (ele, gas, oil, propane...) have changed positions many times in recent years on which is most expensive or cheapest. About 8-10 years ago, ele was the most expensive. Now it's the cheapest.

That said and with the options in your post (ele. can go to tenants, you have an electrician), I'd propose you go with electric. You didn't mention exactly what type of system you would put in (be it central or baseboard) but if there's a way to keep the existing system in there, that may be beneficial. Who knows, in 8-10 more years, oil may again be the cheapest option.

Electric - keep the tenants paying the costs for their bills.

A side note, when I was in University I had a land lord that would sneak into my apartement and turn the heat up. I'm not sure why but it sure pissed me off (they were otherwise nice people). I just disconnected the thermostat after a while.

I'm very close to change to electric. Just waiting to have Central Maine Power contact person call me back to see approx cost monthly.
I have to see how much to lower rents to stay competative
THANKS FOR REPLIES!!!!!!!!!!