Eximo :
I believe they would all be the same regardless of source, only the license might differ slightly.
Windows 8.1 RT (for ARM based processors)
Windows 8.1 (Sort of like Windows 7 Home Premium)
Windows 8.1 Pro (Consumer version of Enterprise)
Windows 8.1 Enterprise (Volume licensed edition of Pro)
Pro and Enterprise are interchangeable, except for the Enterprise features available on a domain.
Windows 8.1 Professional and Windows 8.1 Enterprise are not the same and are not interchangeable.
There is a comprehensive overview of the features that are available here at Microsoft. Some highlights are:
Windows to Go: Run Windows from a portable USB stick or drive
BranchCache: Cache files locally at a site rather than needing to access them over slower site to site links
AppLocker: Control what applications can run on your Windows machines
DirectAccess: Connect to a corporate network or VPN seemlessly
Also available with Software Assurance for Volume Licensing is the
Microsoft Desktop Optimization Pack (MDOP) which provides access to:
App-V: Create centrally controlled application packages that run virtually on clients
UE-V: Synchronize settings for Windows and applications across multiple machines
DaRT: Powerful tools for system administration, like offline malware removal and password tools
MBAM: Centrally manage BitLocker drive encryption
AGPM: Administer management of Group Policy management of clients
Also, yes, the ISOs are the same.