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both games offer a variety of fun and challenging songs; furthermore, both of them come in affordable prices. The main difference lies in the stellar master mode and dance master mode. Both of them are quite similar, mainly because they both enable you to unlock songs/modes. Extreme 2 administers a variety of fun and challenging songs, i.e. cartoon heroes, psmo, sakura, paranoia survivor, etc. whereas supernova offers songs such as max 300 (smmm), fascination maxx, fascination maxx (elm), xepher, and DoLL.Personally, i prefer DDR SuperNOVA over Extreme 2. But, the final choice is yours.

Get DDR. It has a better range of songs, and difficulties.

If you can, get some Dancing Stage games. They are made by the creators of DDR (Konami), and they have lesson mode, where you can learn techniques if your a begginer

I have DDR Supernova and really like it. I've never played any others though so I'm not sure about them.

neither. ddr mario mix is totally the awsomest ever. (=-D)