Thanks for the replies everyone, I appreciate your expertise. Regarding heat pumps - one reason I want to stick with baseboard heating is because it's allergy friendly, i.e. doesn't blow dust around like fan blown air does. I'm guessing that heat pumps don't produce high enough temperatures for baseboards. There's no gas service where we are. I think we'll be sticking with oil for this upgrade, even if it's not the final word in efficiency. Now, if I ever build my own house I'd put my $ into passive solar design and geothermal heat pumps...maybe someday!
I've got all my quotes together. One thing I've noticed is a wide spread on prices. The pricier guys do blower door tests during the installation to make sure they've got the right sized nozzle on the burner, add a water softener to the closed loop, include a boiler stand, fresh air intake, etc. The message I've gotten loud and clear from everywhere I've looked is that the quality of the installation and maintenance support is more important then a few dollars in installation cost. Are those types of extras worth an extra 20% on the cost of the install?
Unlike ducted heat pumps, mini/multi-split systems don't have air handlers & ducts. The most efficient versions have continuously variable speed blowers on the interior units, as well as continuously variable compressors. In an eastern MA climate they will meet or beat geothermal systems on whole-system efficiency whenever the temps are 25F or higher, at a tiny fraction of the installed cost.
The downside is that they only heat the rooms that have the interior unit(s), but if yours is a reasonably open floor plan, keeping the "cozy zone" at 74F with the mini-split may be enough to keep the rest of the place in the high-60s or higher, but this would be a design judgment.
At 10F or lower heating with the mini-split starts to look more like that of the oil boiler, so it's not a disaster if the boiler has to pick up the load at the extremes, but between 15-25F every BTU you get out of the mini-split costs less than half what it costs with the boiler, and at 45F it'll be 1/3 or less- a real discount.
Daikin "Altherma" hydronic heat pumps are indeed pretty inefficent at 120F and above output, so it's not a good match for fin-tube. It can be OK with cast iron baseboard if you have enough of it, but it's really designed for true low-temp radiation like euro-panels or radiant floor slab.
Even the smallest nozzle will still be 2x oversized for your heat load, but the blower door tests in conjunction with AIR SEALING is a worthwhile expense. See if you can't find an insulation contractor who offers blower-door verified air sealing as a service (they're out there).
[edited to add]
Geothermal is highly oversold in this climate, given the current state of the art of mini/multi-split efficiency. For less than $10K you can get a 4-zone 2.5 ton multi (for ~$5K you can get a 2-ton single head mini), whereas a properly designed geothermal system is going to start at ~$25K, and carries some design risk. Even if the well-designed geo gets an average COP of 3.5 when air-handler power is included, and the mini-split only gets between 2.5 & 3.0, for the $15K+ delta in cost you could install grid-tied solar photovoltaics that can make up the difference in power use. When it's 45F+ out a better-class $5K mini-split will usually beat even the best of the geo systems on raw system efficiency.
Key to getting the most out of them is to "set and forget", since they run most-efficiently at 1/3-1/2 load or less- if you turn it down overnight it'll have to run full blast on the recovery ramp, and you lose more in make-up mode than you save by turning it down.