Hi Toni,
1. Here is an article which talked about comparison between App and Outlook COM, you could take it as reference:
https://www.alithya.com/en/insights/blog-posts/microsoft-dynamics-365-outlook-client-vs-dynamics-365-app-for-outlook
2. Comparison between two products has been also mentioned in this official documentation:
https://aka.ms/OutlookCOMPlaybook
From 1 & 2, the most obvious difference for us would be that Outlook COM is built on legacy web client, its icons and UI are not as modern as UCI.
3. In my opinion, due to they are built with different technology, so web pages for them shall be different.
To confirm this, I ran a custom javascript on Email entity > Email form, it will get URL of current page and populate it to description(email content) field.
function getClient() {
debugger;
Xrm.Page.getAttribute("description").setValue("Your client is: " parent.location.href);
}
If I create an email from Dynamics directly, it will display as:
orgURL/main.aspx?appid=xxxx&pagetype=entityrecord&etn=email
If I create an email from App, it will display as:
orgURL/uclient/main.htm?appid=xxxx&clienthost=3&liveid=1&org=crmxxxx&pagetype=entityrecord&etn=email
there are some parameters in URL which tell us web page will be different for different mode.
I haven't Outlook COM client for test, but it shall also has its own parameters because it is based on legacy web client.
Regards,
Clofly