You need to consider the cost per btu of both electric and LP which will require that you know the cost of each in your area.

I can't imagine any reason to use an electric tankless for this application, but gas tankless are used for this application.

the price of lp has jumped up..i would reconsider that route

Rather than guessing, find out the cost of a gallon of LP and the cost per kwh of electricity.

There are 100,000 BTU in a gal of propane
and 3,413 in a kwh of electricity.

You will get about 85% efficiency out of a tankless propane heater and you will get about 100% efficiency out of the electricity.

If you need help figuring it out once you have the fuel costs, let me know.

Jake, thanks for helping me out with this.

LP is $2.06 per gallon
ELEC is 10.33 cents per kwh

Thanks again, I'm just trying to figure out the best way to go. Also, you say tankless is fine, but do you think it is better for radiant heating?

At those prices it would cost:

$2.423 per 100,000 BTU output for propane @ 85% efficiency
$2.168 per 100,000 BTU output for propane @ 95% efficiency
$3.02 per 100,000 BTU output for Electricity @ 100% efficiency

Propane is about 28.5% cheaper than electricity when you use a high efficiency heater.

If you go with a propane water heater that is about 67% efficient (a standard gas water heater) rather than 85%, the cost per 100,000 BTU's will be about the same.

Some of the tankless units include instructions for connecting them radiant heating systems - thus my conclusion that they will work and are designed for that purpose. I would only plan on using a tankless unit that states it can be used for radiant heating.

I am assuming you are doing this on a concrete floor. Do you know what your water temps need to be? If this is low temp you could do this with a condensing tankless heater that would bump the 85% efficiency up to about 95%.

If it is low temp, then I'm also assuming that you are comparing the tankless to a tank water heater rather than a boiler.

The tankless will be more expensive to purchase and install, but will be smaller, much more efficient (35-45% more efficient) and will be able to produce more heat quickly than a tank unit. In water heating applications, that efficiency gain takes about 5-7 years to pay back. In space heating applications, the payback would be much sooner.

Food for thought: You plan to live in this workshop or just use for weekend projects? Radiant cannot respond fast, so you will need to keep it at temp all the time. If you only plan to use the workshop weekends, than you may want to reconsider Heating it all week. I was thinking radiant in my garage, but dumped the idea per the reasoning above.

It's true that radiant heat in a slab will take several hours to come up to temp.

That could be fine for weekend use, but not if you plan on just using it sporadically. I don't know that you would need to heat it all week.