Actually, since the servers themselves are physical assets with a useful life, and indeed could be leased independently from the business model during that useful life, they'd be modeled as depreciation not amortization. But yes, EBITDA would be increased either way.

Realistically, the valuation multiples on EBITDA for mature SaaS businesses are different from those for datacenter operators, and someone building a financial model for Dropbox would now likely take a blend of these this into consideration. So EBITDA is by no means the end-all be-all here.

EDIT: And taxation is a different story altogether, as in either the cloud or own-hardware case, Dropbox can write off expenses or depreciation respectively.