I guess it depends on how much you're planning to stress the system. If you're going to be doing IDS and all that on *sense and have a fast internet connection then you're not going to regret the extra performance of the N305. The only thing that puts me off these tiny cases and the N305 is the extra heat they have the potential to generate. I've seen models completely passively cooled and some than have a small fan. Because I only run it as a router appliance in my sitting room I need passively cooled and so would feel "safer" with an N100. But if I was OK with actively cooled then I'd definitely look at the N305 as the price difference has shrunk a lot and depending on the application you can get twice the performance.
When I ran HA and Z2M containerised on my old UnRaid box the CPU never seemed stressed and that was an old 6th Gen i5 I think. I don't think they'll need the extra grunt. I'm not au fait with Proxmox so no idea how much overhead the hypervisor needs. RAM might be an issue - I think the two chips have different capabilities with addressable RAM from memory if that's important to you.
