I agree with 58Sniper. It's tuff to tell and there are a LOT of factors to take into account. Exchange 2007 can be a pricy investment with hardware and software. Hosted services could be cheaper, but what happens when your internet connection is down and you've got no internal mail? What about when you need to restore an e-mail from a year ago? How about something less concreete, how do you trouble shoot an issue when 1 user is having problems sending email to 1 other company. All your other e-mail is being sent and recieved ok. How do you troubleshoot that? Obviously you have to call your provider but is that included in the cost of your contract with the provider? What happens when your provider comes back and gives you the standard "It's not our problem" response? Now the plus side is you don't have to worry about maintaining it or securing it. Lots of pro's and con's to weigh.
IMHO, I think hosting may make since for a smaller company. For 75 users (depending on your mailbox size) I'd lean towards your own server, but don't take my word for it.