Ground source that has a constant temp are much more efficient.
Ground source heat pumps are more efficient than air-air heat pumps, but require a very significant upfront capital cost in terms of drilling a well and burying tubing, or installing that tubing in some other way (such as in a large pond or in trenches below the frost line). For many applications, it just doesn't make sense as the payback period can be two decades or more.
With improvements in the efficiency of air-air heat pumps in recent years, they make far more sense for many, perhaps even most, situations. They will always perform with a coefficient of performance greater than 1 down to at least their rated temperature, which means they are delivering more heat (or cooling) than the energy they consume because they are transferring, not generating, that heat. Ground source almost never makes sense for a small system or supplemental heating cooling of a small space, and given the current cost of capital (high interest rates) air-air heat pumps often make more sense for even a whole home system, as the extra money you spend on a ground source approach could be earning you money whether you avoided debt for the system, or just invested it if paying cash. Calculating things out on an after-tax basis will tell you what works best for you. For most people, that's no longer a ground source system.
To the OP: Most electric heaters (resistive heat) operate with an efficiency of 100%, so you're not going to get better than that unless you transfer heat from another source, which is what a heat pump (e.g., mini-split) does. Once you decide to go in that (very smart) direction, it's a matter of picking one that delivers the best efficiency at a price point you can afford. There are many threads on this forum that discuss that.
Finally, if you are trying to heat just a very tiny area on a budget and don't want to spend the capital cost of even a cheap mini-split, it's hard to beat an oil-filled radiator heater. They're cheap, safer than a heater with exposed heat elements, and last for years. I heated a tiny 5x7 office I built on a trailer using one, for three winters in Maine. It cost much more in energy costs to do this than if I had used a mini-split, but I didn't need to shell out the $1K or more I would have spent on that mini-split, so I came out ahead in that specific situation. I use mini-splits in multiple other, more permanent buildings, also in Maine.
Bottom line: 1) You can't ignore the capital costs (what it costs to put something into operation) and 2) Almost all of life is in the end a math equation.
Best of luck with whatever you decide to do.
