What surprises many people is that however big or small a portable heater may be, if it they are rated 1500 watts, they all give off the same heat. A watt is a watt no matter how it is packaged. One thing that is different, is whether they have fans on them or not.
As to where to place them, note that almost all forced air furnace outlets and most radiators are locate under windows. This is precisely to counteract the room air that is cooling down from contact with the cold window and falling down. The hot air rises up and meets this cool draft to make it comfortable. That counterflow against the normal draft of a glass surface is why many heaters with fans don't seem to do a good job near a window. The heat does not flow up along the window. Fans are useful where you want to feel the direct flow of the warm air, like in a bathroom.
Always be sure to choose a heater that is protected with an automatic cut-off switch if it gets tipped over. Never put a portable heater too close to a bed because blankets or pillows may end up covering the heater presenting a fire hazard.