With 300 users you can really go either way, technically. It really depends on your end goals.
If you want to get rid of Exchange, then you are looking at what is called "cloud-only deployment". I would recommend not configuring Directory Sync in this scenario, it will almost assuredly cause you problems after you remove it later. I would bulk import and create your users and use Cutover migration in this case.
- It's not that complicated
- Manage users in the cloud
- Exchange Server / Sync Server not required
- You need to recreate or manually copy some things like Distribution groups.
- Separate Logons between local resources and O365
- Manually reconfigure desktop and mobile devices
Hybrid migration gives you more flexibility and it works very well. But this does require Directory Sync be configured. If you are running Directory Sync, you also need an Exchange server around for management purposes.
- Users and Groups are pre-created in cloud
- Usernames/Passwords are same both locally and in O365
- You can move mailboxes back from O365
- Clients that support autodiscovery will pick up changes automatically.
- You really do need to keep at least one Exchange server around for management purposes
- Somethings don't work as you would like (Group Management is a big one).
- You need to Keep Azure AD Connect, and Exchange updated, and the OS they are installed on of course.
- Mixed sources of authority - Office 365 groups in O365, but security and distribution groups in AD (you manage these differently based on SOA).
Exchange purist will scoff, but I usually consolidate the Azure AD Connect and Exchange server onto a single box. At the end of the migration, there are no mailboxes hosted on Exchange, the purposes are now mostly for management, and in a lot of cases to relay mail from internal services (applications, printers, scanners, etc). You may want to check out this resource here: https://technet.microsoft.com/en-us/exdeploy2013/Checklist (if you have trouble with link, search for "exchange server deployment assistant")
If you go the Hybrid route, AFTER migration consider adding in a 2013 or 2016 Exchange server and decommissioning the 2010 server. The interface is closer to Office 365. Further, it can provide a better experience for users that must manage groups if you create a role and grant them access to do so thru 2013/2016 ECP (You can't manage it in Outlook any longer). The alternatives, are command line, ADUC, or manually moving groups to Office 365. See this support article for more info: https://support.microsoft.com/en-us/kb/2417592 (or search for 'Office365 Hybrid group management')