DSM2 is the second generation of equipment from the Spektrum brand. If you buy Spektrum brand, definitely get the DSM2 because
> DSM transmitters will only communicate with DSM receivers
> DSM2 transmitters will communicate with DSM2 and DSM receivers
In their aircraft systems, DSM is discontinued and only DSM2 is being made.

As far as what you should get. Which one feels better in your hands? Which one has the most model memories for when you start adding to your collection?