Let me point your question someone in the Sales team to this question, as maybe if you are using Open Source Edition you can use the 25% of discount, sounds good, and the you can compare.
The Network Edition come with bunch of Features but specially with Technical Support, Backup, etc. Know that you have the Support Phone or email and it will be in the other part a Zimbra Engineer is a must. Also the Network Edition gives you Touch Client, and High Fidelity Preview, and ZeXtras doesn't offer nothing like this.
Also, please note that Zimbra doesn't support ZeXtras in Network Edition - https://wiki.zimbra.com/wiki/ZeXtras:_Zimbra_Does_Not_Support_or_Endorse_ZeXtras_or_like_applications
We definetly need to have the Comparison Table between editions in the main website, not in a PDF that is difficult to find, sorry about that.