Yes, the longer the bubble of CO2 is in the water, the more CO2 will be dissolved. If you watch one of these bubbles traveling up a ladder, you can see that it shrinks as it goes, which is it dissolving into the water. This is also why you want to get the smallest bubble possible from the beginning: large bubbles will move faster, and also have proportionately less surface area compared to smaller bubbles.
An airstone by itself won't do much good. The bubbles that it produces are large, and they'll go straight to the surface.
I've used ladders before, and they work decently--certainly well enough for a moderately planted tank that doesn't require a lot of CO2. You can order the ladders separately online, too, so you don't have to buy an entire system that you're not planning to use just to get one.
Diffusors work even better, because they produce smaller bubbles. They're more expensive than the ladders, but well worth the extra cost.
The main thing is just to keep the CO2 bubbles in the water for as long as possible.
