It really depends on other factors of your infrastructure.
You probably think that it being a java based game/server - means that its operating requirements will be the same on linux and windows.
Windows is fairly sluggish and not responsive with regards to memory handling.
It has so many junk processes that run concurrently that steal CPU operation cycles from the java virtual machine.
Linux is however fairly good.
I think if you are using a Linux based server - you will be able to get away with more for less.
I currently have a FTB Infinity server running - its got 6GB Ram, and it was advertised by the hosting firm as being suitable for up to 120 players.
Based on the amount of contraptions and mystcraft worlds running with chunkloaders and the good performance so far- I have no reason to doubt that the 6GB could handle at least 50% of that load.
Note - I even ran my minecraft server initially in the cloud on Amazon AWS
While the performance there was not as good as what I have now - there are tricks I learned from operating on AWS on Ubuntu instances, that lets you give yourself larger swap sizes which simulates having higher ram allowances (20GB SSD completely dedicated to Swap/Ram). (These are tricks that premium hosts don't want you to have access to)
Ram is important, but we said before - it depends on the amount of chunks loaded per player/chunkloader. 16GB for a minecraft server would only be expected if you are one of those professional servers that have 1000+ players concurrently. I would be very surprised if it takes 16GB of ram for a Ubuntu running a minecraft server populated by 30 people or less.
The slowest operations you will typically find on a minecraft server are the backups and dimension loading.
You will usually get a faster dimension load (when going through a portal) - when the portal on the other side is kept alive by a chunk-loader. (On AWS, mystcraft zoning/dimension jumping - could end up DC me just due to the lag)
The side effect of this is that the dimension does not get backed up when players leave the dimension since the server doesnt register that the dimension is empty - you need to rely on scheduled backups.