Best thing is to do what you're doing.... poke around the net and take it all in. There is a ton of info from owners of the various trucks.
Lots of good info here but a lot of "my brand is the best" BS. Try to check out brand specific sits. For Chevy/Duramax go to www.dieselplace.com
I'm not sure of the Ford & Dodge but I'm sure they are out there. I always figure when doing internet research for a big purchase like this you can make around $100/hr or more just by educating yourself and making a wise purchase. Shooting blind by listening to salesman you are doomed. Get educated and go in armed with knowledge and you'll do well. Good luck.... these modern diesels are amazing.