A splitter literally splits the air going under the truck from the air going over the truck. If your bumper sticks out further than your splitter, that air pushes up on the bumper, negating the effect. The splitter works best when it's less than 2" from the ground, much more and it's pretty useless. While an airdam reduces drag from turbulent air hitting all the suspension components, it also increases the frontal area of the truck, causing more drag.
Lowering the truck, and adding an airdam in conjunction with the splitter less than 4" off the ground is optimum. Either remove the bumper, or recess it to maximize the splitter's performance. You can go with a bigger splitter, but you also want to watch the weight you're adding to the nose. You want the splitter wider than the nose, and incorporating dive planes is recommended. Last, you want strong mounting. You should be able to stand on the splitter without fear of breaking it. I personally like using short steel cables to support the nose of the splitter with a hard mount at the back. If your suspension is in compression and your approach angle to a dip is too steep, the splitter can scrap the ground, and fixed supports will drive the splitter into the surface, potentially ripping it off. Cables will allow it to flex up.
We run an ex-NASCAR splitter on our BMW, attached to the factory air dam. The car's low enough the splitter rides just under the 4" mark. The splitter's some kind of cheap plastic that glides across the ground if it makes contact. It's a little less rigid than plywood, but our setup is very durable and repairable which is a consideration for us. You can notice a handling difference without it. If I remember right, our previous MR2 was about 2 seconds a lap faster at MSR Houston with it.