Personally I put them in a nice thick soft case and bungee cord it to the front rack. When you need it unzip the back 12 inches and slide it out the end. Ive tried both the ways youve mentioned and I dont like them based on how much space they take up, and the fact that trees are still going to rape your gun unless you put it in something (referring to the U shape).
Try the Kolpin cases. They have cases and mounts for Sporting and Tactical Rifles. The nice thing about these is they have a removable soft case inside the hard case. Cabela's has both types around $100. Mike.
Do not put your gun on a U shaped rack on the back rack. No way to tell when it breaks and lets your rifle fall off. Ask me how I know... The gun boot is OK, but you can't keep your bipod attached. The best bet is really a good quality soft case and bungee cords. I also like Gunner75's adaaptation to the U rack on the front.
The simple U shape brackets can be quite strong you can play with them a little before you buy to see how sturdy they are. I have had a set I use for an over head rack and they work fine despite being designed for this type of load.
We also have one 4 wheeler down at the farm with the Kolpin “boot” style gun rack and it always in the way getting on and off and takes the longest to grab the rifle when you want it. I prefer the simple sling if I am just taking the rifle from the truck to a place to shoot.
ditto pgarcia koplin is great...if there is a problem you can talk to a engineer name Jim...he is great and will spend a lot of time on the ph helping you work out a problem...and i think this gets his juices flowing and gives him some better ideas