I’ve got a 700sqft 3y/o 1 story with full basement house. I’ve got radiant heat in the basement slab which I keep at 65 degrees. Typically I heat the ground floor using my mini-splits. I’ve also got radiant floor heat under my hardwood floor. They are installed under the subfloor and have aluminum heat syncs to direct heat upwards to the wood and I’ve got r20 below that to help the heat stay up towards the floor and not down to the basement. The hot water in the house is run by a tankless propane water heater. I live in New England. Figuring nighttime lows of 25 and daily temps of high 30’s. I’ve usually heated the house with the mini splits but electricity just went up 130%. So I’m trying to decide if it’s more efficient to heat the ground floor by continuing to use the mini splits or if I should use the radiant heat below my hardwood floor. I’m paying 35 cents per kilowatt hour and 5.50 per gallon of propane.

Expensive area! We pay 9 cents per kilowatt-hour and $2.50 per gallon of propane.

Under-floor plate system HR is a very low performance approach and reputable companies don't use it except maybe for a client who demands it for a remodel. Slab is best and products like WarmBoard are far better performing than under-floor. As you indicated, you need to place a lot of insulation under the PEX/plates to overcome all the floor R-value above the PEX/plates.

To answer your question definitively and properly, you need to run your fuel costs, heat source COPs, heating degree days and building load numbers using a calculator like this:

https://www.borstengineeringconstruction.com/Integrated_Heating_System_Performance_Calculator.html

A similar subject GBTF discussion may be found here:

https://www.greenbuildingtalk.com/Forums/tabid/53/aff/12/aft/86467/afv/topic/afpg/1/Default.aspx