New Windows 2012 R2 VM install - Dynamic or Fixed disk size ?

I have sued Fixed size Virtual Disks in the past

With very limited test environment I've not noticed any real performance problems

I'm planning a new Install:

Windows 2012 R2 Std - Hyper-V Host & Windows backup rolls only (using BackUpAssist)
Windows 2012 R2 Std VM - for DC & Files
Windows 2012 R2 Std VM - for Exchange 2013
Windows 2012 R2 Std VM - for SQL Server (accounts system)

I see comments that Dynamically expanding disks with VHDX (generation 2) is a good option

Dynamic reduces backup size and time taken

Any advise would be appreciated. 

Relatively small site about 20 users - grow to say 30 max in 2-3 years

ChrisS