Just to confirm what others are saying - the choice of stop condition does not change how things are rendered at all. So in your case, your 25 pass render will be the same as your 5% noise render, as both stop at 25 passes and have done the same thing.
The reason for the option is because you might prefer different stop conditions under different circumstances. Let's say you have an animation, and as it goes through the animation the scene is changing (the camera looking at something else) so if you rendered the same passes of 25 for every frame, some frames will be noisier than others (due to the different objects, materials, lighting, in that particular frame). To avoid that, you can use a noise limit, then each frame will have a similar amount of noise, and if some frames were hard to clean up then they will have rendered for more passes to reach that same amount of noise.
Time and passes are fixed, and will not account for whether a frame is harder to calculate and clean up or not, but of course you can then have an actual estimate of how long it will take to render the animation which you can't have with noise. Thus, if you have to have something to show in 1 hour, you can use passes or noise to make sure the whole animation is finished in an hour, while also knowing that the noise may vary throughout the animation if things change significantly in what the camera is looking at.