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.
If you're going to get one without a timer and decide to get a timer unit for it, ensure that the timer unit will handle the current drawn. (Shouldn't be a problem, but worth keeping in mind)
Ensure you get a warranty and keep the receipt. Some models will have an attachment that allows you to hang a towel close to the fins - can be handy it you want to warm up a towel (or nappy).
To aid in the keeping the wheels in working order, do not sit or place heavy objects on the heater ...