patatrat:

NonprayingMantis:
the main problem with an oil heater is that it takes so bloody long to heat up

This shouldn't be an issue considering the OP's stated use pattern.

beforenightfalls: 
and would probably be kept on most of the time as our newborn baby will be sleeping there when he/she is born.

So, oil heater is your best bet for all the reasons listed by other posters. The downsides to oil heaters are negated by the fact that you will leave it on.

We have proven with physics that all heaters will consume approx the same amount of electricity for the amount of heat put out.

Size - probably a smaller one due to the smaller room size.

All that is left to decide is a brand. My advice - it doesn't really matter. Get a cheap one, it will most likely last long enough - the worst that could happen is the wheels might come off, which isn't a big deal if you get a small one and don't plan on moving it much.