A electric heaters have the same efficency.

However, the mass of the liquid filled MIGHT have more uniform heat and give s certian measure of comfort. But I don't have any experience about that.

The oil will retain heat longer due to its higher viscosity, but as Bill points out, it will cost the same to heat either.

"The oil will retain heat longer"

Yes, but are you sure that you want that?

You want it to shed the heat quickly, into the house, so the occupants are kept warm. That's the point of a heater.

Retaining the heat inside the baseboard heater means that the house will be slower to warm up (but also slower to cool down). So you are choosing between 'slow-and-steady' heat vs. 'quick-response' heat. You should pick the one that you prefer.

It has been a long time since I installed any of these liquid filled heaters, but if I remember right, these heaters had a lower surface temperature than the plain Jane baseboard and they also weren't as prone to making expansion & contraction noises.

That said, I never thought they were worth the extra money, but that is for you to decide.

Tom