Tasar's and NS14's are both as tippy as each other. Put them in the water with their mast up without sails and they will both fall over. But the real key is to be sailing against the same boats, so if your local club is sailing MG's, then get an MG, even if only in NS14 mode.
However, be aware that your son will only last a few years (mid teens) in the front until he grows too tall. Then he will kick you out, move to the back and sail with his girlfriend. Then you and the missus can buy a Tasar and sail together.