To allow our kids to bike with us on some family trips, I need to buy a bike for a 13 year old girl and 15 year boy. Both tall. Bike path type of riding. Would I be better off buying a used Trek 700 ($50) or a no brand name Walmart new bike ($100-$150)? I don't want to spend more than $150 per bike (responsibility issues). Alternatively, what about a used Schwinn or so? Thanks.