Wow, not very good news about Quading. I have a totally different take on the subject. My whole family has quads and we have a track at home that is about 1/2 mile long. Now we are not dare devils on the things but they are alot of fun and I have had many of my children's friend ride them. My kids are now 14, 12, and 10, and we started about 2 years ago. I started my daughter on a 50 cc and she is now riding a 90 cc. The first thing that you need to know is what type of riding that you think that you want to do. That will dictate the type of quad. We like the trail stuff. We all have sport quads which have great suspension which I feel is really important. The larger quads (my wife's 250, my oldest son's 250 and my 400) did not come with nerf bars (they are the nets between the front and back wheel that stops your feet from hitting the ground). I put them on all of them. There is a safety course you can take to show you how to ride and all of the quads come with a limiter so you can turn down the throttle so you do not go to fast at first. The quads are heaver than a motorcycle but it is very hard to turn them over. You can defiantly turn them over when do jumping wrong but you won't be jumping right away. I think that motorcycles are much harder on the body and you can get injured much easier.
Now about the different types of quads. There are many different brands and some can get very expensive. I would stick with the major name brands and not buy the cheaper ones from China. We have Suzuki and one of the 90 cc is a Polaris. We did try the less expensive quads, but we had many problems with it and it was new. The gas filter broke several times, the trans housing broke in half (reverse for a younger child is not a good thing) and the rear wheel came off even though I had use blue lock tight on it. We got the less expensive one because the major manufactures line for sport quads go 50 cc, 90 cc, 250 cc, 400 cc and 450cc where as the Chinese ones you can get 125 cc, 150 cc and even a 200 cc. It is a big jump from a 90 cc to a 250 cc. We thought that the 250 was just to large for our 12 year old. It turns out that it was fine, but I am glad that we started him with the 90 cc first. It is real important that you limit the throttle first.
If you have any questions please feel free to email me.

Ron

PS. The only injury we have had is with jumping. My middle son hit his privates a few times and the other one landed wrong and hurt his wrist a little. He was fine a couple of days later and was jumping again.