A few other points:
Pulling a 8' or 8.5' vs 7' wide trailer is a world of difference. The 7' is much easier on the driver.
Once you pass 14' long, you get tandem axles. There is no debate.
I suspect that leaf springs would be much easier to get repaired if you ever had a problem.
Spare tires seem to have become optional. Get one and get it mounted inside your trailer. Now that we are there, remember that your truck jack and wrench may not fit your trailer; be prepared. (remember that expanding stuff rule?)
The maximum entry height will be less than the interior height due to the back wall coming down from the roof.