Just a guess, but I'd hazard that the standard low-mass resistance baseboards are convectors, designed such that they need a high delta-T between the air & the heating element to convect, so the transfer-efficiency goes down considerably at lower temps. They hydronic or oil-filled versions provide much more heating element-to-air surface area and are probably designed as radiators rather than convectors. Radiators are inherently more comfortable than convectors 'cuz you get that nice warm glow at a distance without requiring the air to be warm. (Reflective resistance-heat radiators are a fire hazard, whereas the hydronic versions aren't as capable of focused superheating.)

I'm not sure what cracking & popping you're referring to- could it be the ticking of the element heating up as the crystalline structure of the metals changes with temp, or are you referring to an electrical arcing at the switching element of the T-stat, or is there something else?

As for the heavy-steel heater, the specific-heat of steel is only about 1/9 that of water, so pound-for-pound you get an order of magnitude more thermal buffering out of water. It takes 75lbs of steel to get the equivalent thermal-mass of a gallon of water- HOW heavy is that steel heater, again? :-)

Thanks, Dana1. This is very helpful -- I really didn't understand the difference between a cheapo convective heater and the more expensive hydronic heaters. Now the question is -- does it make sense to couple a hydronic heater with a fancy (and expensive) thermostat that modulates the power to keep the heat at a steady +/- 1 degree level? Or does the thermal mass of the heater effectively do this for you?

(BTW, the popping/cracking I was talking about is due to the heating up of the element or radial fins,not the thermostat, although I've heard this noise with portable oil-filled heaters as well).

Electric, modulating boiler with outdoor reset connected to Euro-panels and conventional room thermostats. It is not the thermal mass (though wall panels will idle more than fin-tube) but the modulation of delivered water temperature that makes for sustained comfort. See Electro boilers and Buderus panels.