For what it’s worth, by far the “best” DJ software for iOS is DJ Player. It’s made as an app to take full advantage of the iOS advantages, packed to the rim with features and has a lot of flexibility for a DJ to get the most out of the app. The price you pay? It has a pretty steep learning curve, you will have to dedicate some serious time and effort into the app to master it, but once you do, I dare say there isn’t much you couldn’t do on the app that a full DJ software could offer.
Back on-topic: If you expand your setup with the Z1 and add a small PA mixer (the Yamaha MG-06 is at the top of my favorites list right now), you can hook both devices up to one set of speakers and have individual volume control over each device. It also adds two mic channels should you need/want them. That way it’s very easy to switch between the two.
However … the workflow on both environments is significantly different and you need to stay practiced in both. Also (last I checked) you can no longer use your Traktor Pro prepared tracks in the app. This means any work you do on them (beatgrid correction, cuepoints and such) you have to do again on the app.
Personally my advice would always be to have a primary laptop/controller setup (or anything else you want to use) and have either a seperate mixer with a backup source (iPad with any app, a core collection and no controller) hooked up, or your backup source connected to the aux input of your controller (adding the controller to the single point of failure list, if it breaks your backup won’t be of much help). A true laptop crash isn’t too likely, assuming you take your DJ-ing seriously and invest in good (used if need be) quality laptop, and most likely will be a software crash requiring only a reboot. While the reboot is underway (having an SSD instead of a Hard Disk minimizes reboot time by a lot!), you can easily play a few tracks from the iPad to keep the music flowing. Once your controller/laptop setup is working correctly again, you can effortlessly move back to your main setup (one more reason it pays to learn manual beatmatching).
Again, just my 3 cents.
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