I am very happy with my '03 305 HP truck. I put the EZ on when I'm pulling the big fifth wheel. Otherwise, I leave the EZ off when empty. The 305 has plenty of power to spin the wheels, and in the rain I found I had to be very carefull with the EZ installed not to have the truck swap ends on me.
I see nothing wrong with the '03/'04's and if you can get a really good deal on one I'd go for it.
As far as mileage is concerned, I get between 19. 5 and 20 MPG empty when highway driving, and 17-18 MPG in the city. Towing is a different deal. I've seen as low as 9. 0 MPG and a high of 11 MPG. Keep in mind that I'm pulling a very big rig that catches a lot of wind and weighs a bunch. You would do much better.
Personally, I think you'll do better at resale with an auto, and you don't need the stick for what you are pulling. If your future plans include something over 10,000 lbs I'd go with the stick. Otherwise I think you'll be happier with the auto. IMHO.