Hi @Anonymous ,
First, it's always a good idea to get licensing advice from a licensing specialist or Microsoft sales person - most advice here would be considered informal and licensing can be very confusing.
That said, I find it helps to consider licensing from the user perspective in terms of how many premium apps they are likely to use. At the current pricing model, per app is $5 per app per user per month - per user is $20 per user per month, regardless of the number of premium apps they use.
Based on this, it makes logical sense to use per app when your user will use less than 4 premium apps.
4apps * $5 = $20, which is the cost of a per user license - so if the user is expected to use 4 premium apps and up, it's generally considered better to go per user, as the cost then becomes fixed at $20 for that user but for an infinite number of premium apps.
Hope that helps,
RT