One of the really great additions to Wave 15 Office 365 SharePoint for all plans was the ability to add more SharePoint space beyond what you receive allocated per user. Where did I get that from? Here:
Having just revisited that page after speaking with another reseller who had issues attempting to add space I note the above page says the following for P plans:
Additional storage for Small Business plans is not yet available.
and M plans:
Additional storage for Midsize Business plans is not yet available.
Problem for the reseller is that their customer needed greater space than the default and being told that it could be added went down the M plan route for the customer because of the cost savings over an E3 plan. Trouble is now that they need the additional SharePoint space and it can’t be added ‘yet’. That didn’t sit well with the customer or the reseller as the only option available with the space is an E3 plan and migrating from an M plan is not a simple upgrade of plans.
I am seeing a lot of confusion around what each plan does and doesn’t include. My recommendation is always E plans due to their flexibility. I would also recommend that if you go with either P or M plans check VERY carefully as to whether it exactly fits your needs now and into the future. Don’t rely on someone else, check it for yourself to avoid situation like the above.