Actually, Birdy, the app is convenient, I agree (I have the DTX700 Touch app too), but there is nothing you can do with it that can't be done directly with the module. And since the menu in the module is initially not so easy to understand and navigate in, it become very easy once you did it one time or two. I am not sure that the app should be a buy or no buy element... in addition, did you already consider to use a VST to provide the sounds and use the module as a trigger? If you have a good enough computer, you should try it. Actually, no module can compete with a good VST sound-wise... I was very happy with the native sound of my DTX700 initially... but I understood that it was not so good once I tried AD2.
Your DTX402 have a USB midi-out port that can send midi informations, so you can try it yourself. And the DTX500 have a Midi out port and can then be used with a VST library. It would be a cheap solution to both increase the number of inputs for your drum AND the overall quality of your sounds.
In this configuration, I NEVER use the DTX700 touch app anymore.