You are sort of all over the map here. You just got the Stratos and now you want to sell it or try and make it more powerful. To make it more powerful is going to require more than just bigger motors. You will probably have to change the esc's too and spend more money doing so.
Keep it and fly it as it is but if you don't like it then sell it and get something else.  If you get the super cub I don't think you will be happy with that for very long either.
I gave you my recommendations via PM but that is going to cost a little more than you want to spend . Maybe save up and try and buy a good radio like a DX6 or 7 and then you can get  BNF planes from Parkzone .  Not sure what else to tell you at this point.