This all depends on what you want from a shower, as an example:
Electric showers are fully independent and work from mains fed cold water. The pressure of Electric showers is determined by the KW output, the higher the KW the better flow of water. The advantage of electric showers is that they are fully independent of your hot water system. Example, Aqualisa Electric Showers, Redring Electric Showers, Triton Electric Showers.
Gravity fed showers work on gravity and the flow of water is determined by the height of the cold-water tank, supplying the cylinder.
The bottom of the cold-water tank needs to be at least 1 meter above the height of the shower head to obtain a reasonable flow of water.
Example, click on this Aqualisa Aquarian Mixer Shower these type of showers supply a much better flow of water than the electric showers using the water from the stored hot water system.
Pumped showers, i.e. power showers work on Gravity systems only, there are mainly two types of power showers, a power shower that has a built in pump, Aqualisa Aquastream, Triton Power Shower, Bristan 1000 power shower. And power showers that work with a separate pump that is fitted away from the shower unit, example Aqualisa Quartz Digital Shower, Triton Satellite Digital Shower. These types of showers when it comes to flow of water are the best, this type of shower is usually preferred due to the high flow of water pressure, but the only consideration to make is that these showers use much more water than gravity or electric showers.
If you have a combination boiler or high pressure system you have the choice of ether installing an Electric shower or a thermostatic mixer shower. Example Aqualisa Midas Shower.