depends on what you want feature wise.. if you want to do it inhouse then you have to take into account,
hardware/warranty
software
backup/restore
power/ups
bandwidth
hosted exchange comes with everything, you just pay per user, but you do have a limit on space/email addresses
rent a dedicated server, completely depends who you rent it from...
as a company i support/provide all three, internally when we rent dedicated hardware unless the customer specifies they want physical hardware we just create a virtual machine and we include our support with it, so its more flexible than hosted but probably would cost more too...