Ok, back to square one. I have used so many Dahua and Hik products that I have lost track. The reason why I use them time and again isn't because of their ease of use. That would not be one of the features of either brand. Playback and investigations are very tedious on both brands. I don't know if either has an edge on each other because they both are terrible at playback IMO.
More current models are able to keep up and haven't had as much buffering issues because they generally have faster and sometimes multi core CPUs now. In the past it would take 10-20 seconds to fill the timeline with motion events. When you want to find something quick, it is frustrating.
Now let's assume you find your event and want to preserve it. There is no way that I have found to "bookmark" that event and keep it from being overwritten. You have to export that video to another device or media. Thumb drive backups on the Dahua (when using motion recordings) nets you a potential for many independent files for each section of motion detected and again separate files for non-motion sections. You could end up with 20 video clips for a 5 min event for just one camera view. Try playing that back later and see how much hair you have left. Mine is all gone for sure.
Next you should try their CMS software on a PC instead of directly using the DVR/NVR. This software will be clunky and very hard to use when compared to quality VMS software. They use terms that just don't make sense. There is one main page for live views and playback is buried on another page, which usually restricts you to just four camera views for playback (the newer CMS allows up to 16 views but crashes all the time). Then when you find your event and begin to download it, it is much slower than the USB local backup. Usually you wait 10 mins or more to find out you forgot to click a checkbox and have to repeat the process.
I have seen this process take HOURS to retrieve five minutes of usable video. It is painful and really tests your patience.
That said, when I commend their quality, I mean they just keep running and I have rarely had a product die. In fact, I'm sure I've had one die, but I can't remember when. They do a great job of recording 24/7/365, but just don't ask to get that video exported.
We now ask our clients how often they plan on using playback. If it is often, we will recommend using DW Spectrum as the VMS. If they are cheap or don't really use playback too much, we will use a DVR/NVR.
I hope that helps.