Alsvha is right, this is a huge topic.
I'd consider refining your query. For example, you ask for the "best (most efficient)" 3-tier architecture. But what does that mean? Efficient could mean:
1. Least latency between browser request and server response.
2. Most responsive user interaction.
3. Quickest, cleanest, and least bug-prone development effort.
4. Least content management effort.

Returning to XML vs .NET.
For item #1: this kind of performance generally turns out to be as much implenetation than architecture dependent.
For item #2 above, XML is a component of AJAX, which is intended to reduce the need for POSTBACKS as part of event handling...and ASP.NET supports AJAX, so you can use XML with .NET.
For item #3, I vote for ASP.Net. It sure beats unmanaged ISAPI filters for ease of development.
For item #4, XML is often used to move content between a database and an HTML stream (XSLT, as mentioned in a previous post, is one way of transforming XML that was generated from a SQL Server query into HTML).
You can do this with ASP.Net.

Stephen Lowe
Disclaimer: though I work for Microsoft as an Architect Evangelist, my opinions (and occasional errors) are my own