I have not looked at them side by side. Consensus would probably be that Conquest is a bit better in low light, but I doubt that the difference would be significant. Besides, since you are comparing models that are not identical it is hard to tell much without looking at them side by side.
It may be a more direct comparison if you look at Euro Diamond 1.5-6x40 versus Conquest 1.8-5.5x38.
Burris is a 30mm model which some people prefer and it is available with a #4 reticle with an illuminated dot which can be helpful in low light.
Conquest probably has slightly better glass.
Pick what's more important to you.