It is doubtful you would be disappointed with the diesel. I've never heard anyone complain about having too much power or towing ability.
The biggest thing people lose track of when doing a car deal is that it is a business deal. The car sales people will try to make it personal and get you "invested" in it. Just stick to your guns. Personally, I only negotiate an out-the-door price. In the long run, at least the way I look at it, that is what you will pay regardless of what fees they want to charge, etc. I always have financing arranged before hand, but still give the dealer the opportunity to beat it with the same terms.
While many forget, you have the power in this deal. It is your money and there are many other places you can go to spend it.