EDIT: My answer is clearly not an answer. So here is an edit. The Microsoft coding guidelines state:
Do not use a prefix for member variables (, m, s_, etc.). If you want to distinguish > between local and member variables you should use “this.” in C# and “Me.” in VB.NET.
Can be found at: http://blogs.msdn.com/b/brada/archive/2005/01/26/361363.aspx
So it would seem that at least from MS there is no clear guideline, although another answer does state that StyleCop makes it a guideline. There is no authority on these things, so I would suggest you make up your own mind, or in this case give in to your team. It's not such a big deal.
My original answer
I personally agree with you, but maybe a reading comprehension test pitting the two methods against each other would be valuable. Otherwise these style things are just mudslinging.
My salvo to follow:
My opinion is that people are unnecessarily complicating their code style, and if they need to indicate that something is a class level variable there might be some other serious structural issues in the code, like the age old recipe-method of putting private variables at the top of the class which force you to constantly scrolling up and down.
this strikes me as one of those "what this is" conventions versus the correct "what it does" naming conventions. Brevity should be favoured above being explicit. This is a lesson that is repeated often by dynamic languages. We don't need all the fluff!