Hi Carl
Moving users to O365 doesn't mean you need SSO.
Dirsync means you have to enter the credentials again on the O365 portal (although the same as on-prem AD) where as inside the Corp LAN, you don't have to do this when using O365 services like Exchange. It will use the AD logon you used on the WIndows PC.
There are trade-offs with this, AFDS requires more investment that dirsync. ADFS is a true single sign on. There's other features with ADFS which customers use, such as claim based rules
I'd recommend a good read on ADFS before committing to it.