they use the same sensors usually so neither will have the 'best' night mode, if anything separates the two's capabilities its due to the built in IR.. I could take a dahua with LXIR and it'd be better than a Hik mini-bullet w/round diodes.. and vice versa.
for starters Dahua will check your model against a list of compatible models contained within a firmware image before attempting to upgrade; so if you have a firmware file thats not for your device.. it will reject it instead of bricking its self... its got a better ONVIF implementation and works with 3rd party systems better than Hik.. Its Smart features like Linecross are alot better, there clients are better supported on Mac's, etc.
and most importantly, there is no region locking nonsence.. North American, European, Chinese-Only, Asian Market devices will all play nicely with each other regardless where you aquired them... cant do that w/Hik.