Hi Tigros,
Welcome to the forum!
Out of the three you are looking at the RL500 will win ever time!
The Hammerhead 250, is a little on the under powered side, not bad off road but on the road it's quite slow 50mph flat out
The Quadzilla RL300, is a little bit better, stronger machine, capable off and on road, 55mph again flat out
Th Quadzilla RL500, is a very nice piece of kit, high, low and reverse gears, 60 - 65mph. Gets to top speed quite quickly, very good off road. It's tyres, suspension and brakes are far better. The engine is the same that they use in the 500cc Quad bike, it's very strong and smooth running through all areas of the rev guage.
Good luck with your search, let us know what you buy and get some picture of it on the forum!
Steve.