If there is a difference between the cost of a gas shower versus electric shower (which would vary depending on the setup) any saving would be completely outweighed by the cost of changing the setup.
The only reason I was asking was I have both, the only cost would be some sort of adaptor for the mixer tap.
My boiler isn't an efficient one but I don't use it much.
Hopefully it's as they say and prices will fall again next year.....you know, like when diesel prices fall loads once they've risen sharply.