I5.

Gives you a few more cores to play with down the track if you need them plus better performance from the get go.

Do you plan having VMs or only Containers? Because Plex, alla ARR stuff and NVR can work on containers. In that case the i3 is extremely overkill too, but if you plan to run a lot of VMs, the i5 of course have more core.

In terms of power consumption, both idle around 25W (generic system with no GPU and Idling HDD), the i5 would peak more, but depend on the usage, generally server idle 99% of the time.

TDP doesn't mean Power Consumption for Intel, but is related to Thermal Dissipation. The i3 would consume the same as i5 idling, because of C-state and the ability to almost shut off the unused cores, then on peak power, having more cores and frequency, mean more power consumption, but peak mean burst of 1 second at max.