A heat lamp would be by far the cheapest and easiest to install.
I did electric in floor heat in one of our bathrooms and the warm floor is great, an infloor thermostat gives better control.
The heated ceiling panels of course have the advantage of not being bright or oddly colored like a heat lamp.
So it really depends on what you want, what your budget is and how much you want to rip in to the bathroom.
I would start out with a heat lamp bulb in the ceiling.
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