If you've got a small business and want to move to Office 365 or to Google Apps, the main choice is whether you want 1TB of storage per user (Office 365) for $5/user/month or unlimited storage (Google Apps) at$10/user/month.
We are going to try Office 365 as we've not used this system too much and many people we advise are more likely to go with Office. We are assuming we don't need Office licenses for this, so this is just email and storage.
In any case here are the nuances of getting this working. I won't cover the simple sets which are basically signing up and then changing your DNS to point to validate you as it varies a lot depending on your DNS provider. And there are some good migration gotchas from folks like Infoworld

### Partial Exchange Data migration

Office 365 let's you move a few users at a time so you can run in mixed mode. Perfect for having say the IT guy move, then move a few more and see how it goes. It also allows easy fall back if necessary as you can leave your old mail server running and just change the DNS if Office 365 has a cow. So first you need to logon to office365.com and then go to Admin/Exchange/Migration and click on the + so start one and choose migration to. There are many flavors, but we will use the simplest one which is IMAP migration since basically everyone supports it.
Tip. The strangest part of this is that it requires a CSV file with the user name even for a single user. So many people at Microsoft and there is no online form? When you click on the help, at least in Google Chrome, you get nothing, but the CSV is kind of what you would expect. If you have Windows domains, then you use domain\user as the syntax. If you are using a hosted server, then it is probably more like rich@domain.com, so the file needs to be like this with the headers exactly as shown. Of course the big problem here is that you need to know everyone's password (yuck!) or have an system-wide admin password that gives you access to every mailbox. The Exchange specific options assume that btw:

EmailAddress,UserName,Password