In your opinion how should we proceed with a customer who is asking you to give a sales presentation when he/she is sitting with your competitors?

My advice is to deeply study the product you are selling + study weakness of your competitor related to your products + points you have strength + points you are sure customer will be interested in.

Then talk and present.

By showing our products,services and quality atmoshphre on which ground our competitor does nt have.

I would show him the Competitive advantages of our services and/or Products of our Company so He will change his mind about the our company, that will make him leave the competitors and join us. 

Comparative anaylsis is the only way to make difference of your Product before your competitors but one thing must be keep in mind that he/she will arise many cross questions n you should have to justify logically.