Oil or LP gas

Gas isn’t more efficient than oil. There are gas boilers that may claim to have have a few percentage points of efficiency higher.
These are super efficient in gas or oil.
https://energykinetics.com/
If you know how many BTU'S you need to heat your house, you could do the math based on your local gas rates.
Don’t forget to include all the charges the gas companies charge every month for your total costs per btu.

For most people LP is not competitive with oil - but if you own your own propane tank and use enough propane it can be close.  If your electric rates are high, using propane for cooking and drying clothes can help the math a bit.

I have no real difference with your comments, @Solid_Fuel_Man -- so far as they go. However. First, with similar or the same return temperatures, and equally well cleaned and tuned boilers and burners (and the burner properly matched to the boiler -- particularly, with oil, the nozzle geometry), oil and gas will be within a percentage or two on efficiency. Now if you have a system which is designed so that the return to the boiler can be at a low enough temperature, then you can indeed gain efficiency -- sometimes in good situations as much as 10% -- provided you are firing with gas. While the stack gas from a condensing gas burner is rather corrosive, most of the nastiness is taken out in the condensate. With oil, even low or ultra low sulphur, condensed stack gas is truly agressive, and over the life of a boiler you will be better off not allowing it to condense.

So -- on your well designed and well maintained radiant slab applications, I certainly would not argue against you (although the magnitude of your improvement may be partly due to other factors). I very much agree that for a radiant slab application, a properly setup mod/con will have higher efficiency.

But what about other forms of radiation? It may be very difficult to get enough heat out of the radiation to satisfy the structure needs and still have return temperatures low enough to condense, at least for the colder months of the year. Then the gains are somewhat less, if present at all. And, of course, with steam there isn't enough condensed steam to bring the stack gas temperature down significantly (Cedric, for example, produces a whopping 56 gallons of condensate in an hour -- compared with a hot water system, which may be circulating close to that per minute for the same output) -- so your maximum efficiency on steam or non-condensing hot water will be in the 83 to 86 percent range, whether oil or gas makes no difference.

Now the economics. A very important thing for @Rock to consider is the cost, both capital cost of installation and running cost. Capital costs aren't that hard to figure. Running cost however... the approach there must be to compare the cost of the fuel in terms of BTU, not in terms of gallons or cubic feet as the case may be. LP has a heating content of roughly 90,000 BTU per gallon. Fuel oil has a heating content of 140,000 BTU per gallon. Thus, it will take half again as much LP to heat your structure as oil -- and this must be taken into account in the pricing.

LP pricing seems to vary a lot based on your location, and the amount of competition in your area.  Where I live LP is normally not competitive with oil unless you use over 1000 gallons per year. 

Correct, LP has less "heat per gallon" by about 1/3. So a cost per BTU as well as the cost per deliverable BTU all play into the decision. 

Stack losses with the burner running, and standby loses when it's off are much higher with oil, Energy Kinetics is really the only one who addresses this to any degree with oil. 

As far as condensation in a heat exchanger, I'll wager the true life costs of a mod/con and a standard oil boiler would pen out quite similar. Wholesale cost for a mod/con and a comparable oil boiler are pretty much the same. When venting is considered into the equation, oil is far more expensive as far as capital cost. 

I have several mod/cons oit there which are approaching 15 years old, with few to no problems. Annual maintenance is less than that of oil, but an expensive component will put it on par with oil for annual reoccurring costs. 

Overall the math works out very favorably for LP in my region even with the shorter lifespan of mod/con equipment. I just like the whisper quiet operation and plastic sidewall vent options of condensing equipment.