As far as I can remember we have almost exactly the same server setup and I'm not looking to change it anytime soon.
Even 1080P HEVC files play just fine with the G3258.
But I think it's going to be either or really with the i5 more than likely being underutilised in both situations, also it's the same chipset and everything so swapping them out will take 5 minutes without reinstalling any software/drivers if at some point you think the server needs the i5.
Also if you feel the G3258 starts to struggle you can easily overclock it by another 1GHz.
Yeah thinking more about it I'm just going to keep it the same way it currently is. I'm sure later down the road my daughter will utilize the i5 more than the Plex Server.