Dynamics CE instance planning

If you had different applications which could all be implemented in Dynamics (maybe different flavors of the case/contact management), how would you decide whether to put those applications in the same instance or whether to set up different instances for some of them?

Not sure if there is a simple answer.. My colleague and I spent a couple of hours brainstorming this question earlier today, and it does not seem that we came up to any conclusion yet, so, if you have any thoughts, you are more than welcome to join in

To start with, why would we want to have a separate instance for each application?

• We can have clear data separation – there is no risk data from one application will surface in another application
• There is no need to worry that configuration or customization changes made for different applications will start interfering with each other. This applies to the business rules, workflows, plugins, scripts, etc.
• When the same entity is added to two applications in the same instance, corresponding entity views may show up for each of the users. Well, we can still control which views will be added to which apps, so, at least in the UCI, there is a solution for this. Same with the forms and dashboards
• There is still a question of field labels and translations – what if two applications end up using slightly different vocabulary? Those labels will be showing up in the column titles and/or in the advanced find
• Duplicate detection rules may become somewhat complicated, especially when some users have access to the data in one application, and others have access to the data in multiple applications
• Regression testing is more complicated when the instance is shared – when updating one of the applications, we may have to run regression testing of all other applications
• Every instance can have different dynamics portal (well, portal add-on may still have to be purchased)

On the other hand, why would we use the same instance?

• All those complications aside, when it’s the same instance we don’t need to buy an add-on instance(or a set of add-ons to cover dev/test environments as well). This also applies to the portal add-on
• Data sharing may have some benefits (for example, rather than having duplicate contacts in different instances, we might have just one contact record)
• Added on Dec 11: If a user has access to more than one application, outlook synchronization for tasks, appointments, etc will work only when those applications are in the same instance ( as Lema mentioned in the comments below and as Joe Gill mentioned in his twitter reply )

Just by looking at the above(and I’m sure there can be other reasons in both categories), I am wondering if it would it be fair to say that, once two applications have different data models and/or data governance rules, they should certainly be separated on the instance level? In other words, an application in Dynamics seems to be more about working with the data than about the data itself(therefore, if the data is different, there should be different instances, but, if it’s the same data, it could be the same instance). Even in terms of cost-efficiency it might be better to invest into extra instances in this case since we’ll be saving on the extra work.

Or is there something I’m missing and there are other ways to think of this?