What is the difference with Unity free and Unity Pro?

I would like to know from a developers point of view what the difference will be with using the Unity free version as is currently done and the Unity Pro version as is planned.

What will it mean for Project Ion on the short and long term?
Guess there are a lot of advantages in that Pro version but could one of the developers explain to us what these are?

Thanks in advance!

This question deserves a proper answer and I need to reserve some time to write one a.s.a.p.

Hi
There are many differences, some important, others less.
 
You can find the complete answer here: https://store.unity3d.com/
Click the link "Compare" and and you will see.

Nice find, also good all options show some information to what it means.
Still would be nice to hear from the developers what it could do for Project Ion.

Alright, here we go. These are some of the Unity Pro features that we can put to real good use for Project Ion.

Light & Darkness
This is some serious eye candy right here. The lighting features and optimizations are very limited in free so this would be a big asset for us to give a(n even) better look and feel to the game. With this we can also implement a convincing day/night cycle which is actually disabled in the currently used skydome. Combined with the pro feature Level of Detail this would allow for a much bigger view distance on an even better looking world with lots of content on screen at the same time.

Optimizations
LoD, static batching, light probes, occlusion culling, deferred rendering etc.etc. The free version doesn't offer a lot of the built-in optimizing available in Pro. This is a major handicap when you're building one big world where people can interact with each other. Even though working around these limitations was beneficial to our insight and knowledge about these techniques, it does take away time from developing game content.

Eye & Ear Candy
Features like the audio filters, full screen post processing and RenderToTexture aren't actually needed when your working on game mechanics, but as soon as those are in place it's time to polish things to shine. The first two combined will allow for some nice intoxication effects on our boosters while the RenderToTexture functionality is almost essential for the performance of the UMA tool.

Native Code Plugins support
Being able to compile these into the final build makes nearly anything possible. Might be a good thing that we didn't have access to this feature because it offers such a vast array of tinkering possibilities...

There's a ton of other features besides the ones mentioned above, but these are the ones that we really like to get our hands on. Planning the implementation of UMA in our next development cycle I expect the lack of RenderToTexture will probably hurt the most. (Fingers crossed as confidential sources told me that there's ongoing discussion within Unity to let (parts of) this feature seep through to the free version...)