I'm a newb and I'm looking seriously at the Specialized Rockhopper. Test drove it and the GF Marlin and I liked the more upright feel on the Rockhopper. I'm about 5'10" and 160 and was riding a 17" frame. Does that sound about right? If I decided to later upgrade to disc brakes what can I expect to spend on that?

On another note my son is 7 and wants a new bike to ride "off-road". Would a $99 Target bike be ok or should I spend the extra $$ (around $220) and get him the Trek MT60?