G'day Roger!
to IIS!
I suggest you start with the 25mm. It will concentrate the image and will give a wider field of view and allow for a larger overview of the object. Of course, this depends on the size of the nebula, but start with the 25mm, then switch to the 8mm as it will help darken the background and help with contrast, but the trade-off is a smaller field of view due to the magnification.
Something like a small planetary nebula a 25mm EP won't help you very much, but it can help make it stand out so you can identify it. You can then switch to the 8mm.
What scope do you have? Also, from where are you doing most of your observing - urban skies or dark?
Have you had the opportunity to spot the Lagoon Nebula (M8) and the Triffid (M20)? Both are very close to each other, and pretty much dead overhead at this time of year. M8 is better with the 25mm, and as M20 is smaller, it does well in the 8mm. But try both eyepieces and see how each nebula responds through each EP. Don't be affraid to switch between EPs no matter what you are looking at!
Alex and Karl, thanks you for your kind words
Alex.
Alex.