I have two boys who will be 4 and 7 at Christmas and I was thinking of getting them a Hexbugs set as a joint present. There are a few reduced sets around on the Entertainer and Bargainmax - a Raceway set, Space station, Elevation set and Playground set. I have no idea which to get and what the difference is between Nano and Nano Flash. Also would it be best to get some extra Hexbugs too? I'd appreciate any help in choosing!