.InstanceUuid≠ .ExtensionData.Client.ServiceContent.About.InstanceUuid

Introduction 

connection.InstanceUuid ≠ connection.ExtensionData.Client.ServiceContent.About.InstanceUuid

Let's connect to vCenter and check the two:

if (! $creds) {
    $creds = Get-Credential
}
$connection = Connect-VIServer -Server '<vcenterFQDN>' -Credential $creds
$connection.InstanceUuid
$connection.ExtensionData.Client.ServiceContent.About.InstanceUuid

# Results in
# 9058ed3c-1a5f-49e4-8e59-7d24fca84a0f
# 9058ED3C-1A5F-49E4-8E59-7D24FCA84A0F 

So, in one of my environments, one is with lower case, the other is upper case.

Who cares?

Well, if you look at the URL of the webclient, it uses that UUID to identify the vCenter. And it is CASE-SENSITIVE:

So if you're building a tool or something that generates links to the object in the vCenter client, use .ExtensionData.Client.ServiceContent.About.InstanceUuid instead of .InstanceUuid.

I said it is in 'one' of my environments. Most of them are in lower case. I guess at some point in time in the past, one of the versions of vCenter, initiated with upper case and it stays the same, update, after update. So it's not a huge issue, but it's better to be safe then sorry...