I think the costs over 3 years would be extremely similar, after that point it would be marginally cheaper (£3000? £4000? over the 2 years before the box needs replacing) to host it in-house.
If it's for a small business, I would say there's a strong case for 'wait and see' by going with renting services.
If in a few months (or years) it becomes apparent that there are definite benefits and cost savings to be made from doing it another way, you can move down that path. It's also important to take into account what usage patterns will be, attachment sizes, email filing etc.