You ask a question, then you kind of seem to stand up for mods, like plugins stand no chance in this thread.
I vote plugins. Why?
Because you can modify quite a bit of the server. I understand you have limitations as a lot is still handled client side, but you can still do quite a bit.
Your players do not require any mods. For a first time player, mods are very convoluted and very confusing. The first time I tried joining a modded server I couldn't figure anything out. After a while someone told me about the Technic launcher. That made things easier for sure. I joined a couple servers that had a CRAP ton of mods, to the point it lagged my client so damn hard. (1 ex: I was helping someone with some scripting stuff in their mod pack, they had (no joke) 150+ mods. My clients lag was thru the roof)
I don't know enough about the modded community to really give an opinion on it, but it seems like they update slow. Forge for example just came out recently with a working 1.13.x version, just in time for MC 1.14 to come out. Forge for 1.14 just came out about 7 days ago. If we were to compare Spigot, they came out the same day MC released 1.14.
Spigot plugin based servers make it a lot easier on players.
I totally get there are players out there that understand modded servers quite a bit more and find it super simple, but a lot don't.
Personally, I would love to give modding a try. I am doing quite well with the Spigot/Bukkit API, I find it relatively easy. I took a peak at the Forge API and it scared the heck out of me (haha) but then again that's probably just because Im so used to Spigot now that anything else seems so very different!