I'd do 500, just because it "sounds" brighter. /ubbthreads/images/graemlins/wink.gif
Can't go wrong with either one. 400 will run long and cool. 500 is about as high as I'd go if I were using it for much constant-on time. If you use it lots, go with the 400.
The NG400 is really around 430 I think and the 500 is a bit over 500 as well. I pretty much agree with Darell. It isn't an issue of too much heat as much as economy I think. /ubbthreads/images/graemlins/icon3.gif I use most of my lights on low as much as high so I am not so concerned about the high level. I have settled on 500 because of this but if I was planning on using high a lot, I would consider 400.
Low is effectively DD, yes. With the same resistance, the level of low would be the same for a NG400 or a NG1000. High drive current makes no difference. The NG can only go a bit above 3Vin - rechargeables are OK.