A splitter will only work to its full potential if it attached solidly to the chassis of the car, it will just bend the bumper and not provide the designated downforce it is designed to make if its not.
It might help direct a little air up into the intercooler or radiator but thats it.
A diffuser will work when designed right. The "off the shelf" ones you can buy are pretty useless.
Splitter with a flat floor feeding a diffuser will help.