EWS Managed API Should I use GUID or DefaultExtendedPropertySet.PublicStrings while constructing DefaultExtendedPropertySet

I am developing C# .NET Framework 4.5 Windows Form application using EWS Managed API 1.2 with Exchange Server 2007 which performs some sort of syncing of mails.

Now that I am dealing with Extended Properties, I want to be clear some things:

Q1. What is the purpose of DefaultExtendedPropertySet class? MSDN says "Defines the default sets of extended properties."

• Is it just to group the extended properties?
• If yes, why is the grouping there at first place?
• Do we have any Ews API method which can fetch values of all extended properties belonging to the same group on an item?

Q2. I am unable to decide whether should I use custom GUID or DefaultExtendedPropertySet.PublicStrings while constructing ExtendedPropertyDefinition:

ExtendedPropertyDefinition MyXProp = new ExtendedPropertyDefinition(
                     DefaultExtendedPropertySet.PublicStrings, "MyXProp", MapiPropertyType.String);

OR

Guid MyPropertySetId = new Guid("{C11FF724-AA03-4555-9952-FA248A11C3E}");            
ExtendedPropertyDefinition extendedPropertyDefinition = new ExtendedPropertyDefinition(
                     MyPropertySetId, "MyXProp", MapiPropertyType.String);

• What are the factors that should dictate the above decision?
• Also what difference it makes by above two approaches?