Would a model render faster if it has the double sided polygons made using geometry, or by applying a shader?

remember that your targetmachine must have shaders if u use the cullmode thing

i prefer doing it with shaders, its pretty fast

I'm not 100% sure but I really think that just setting cullmode to NONE (it was NONE, right?) would be the fastest (and also easiest) way.

If I understand you right, you want to switch off backface culling (Cullmode = none). This will reduce the frame rate, and only makes sense for a 'flat' model, like a billboard, when you want both sides of the model polygons to be seen.

This has nothing to do with shaders and thus the target machine does not matter.

thanx for clearing this up. but i dont share your opinion on the use of cullmode. think of crystals or water, or did u have a look at my little demo, without cullmode=0 it wouldnt look how it looks

Sure it makes a difference for transparent objects aswell.