The 2108 is an interesting piece of kit, due to the AI. The downside is that it's very limited with AI-channels.
I (always) recommend the 5-series as it's so much more competent. I ran the 4-AI-series before upgrading to the 5-series.
If you can see yourself running cameras tracking plates or PTZ with video metadata etc you definitively want the 5-series as 2- and 4-series lack a lot of functions.
Depending on your setup built in POE-ports may be redundant. If you will run cable from the NVR to each camera, then you should have the POE-version.
As I have cameras on various buildings I utilize my already existing LAN and do not use the POE ports from the NVR.
Edit: In the end of this video, from 1:30, you can see how Video Metadata work on the 5-series, you get LPR "for free". 4- (and 2-)Series are no way near this. The camera in the video is a 7442, all "big" PTZs also support Video Metadata with the same result.