At the present time a lot of the functionality in the Classic app is not yet available in the Connect app, and everyone who has the Classic app already has been told to keep using it until they get an individual message from support to switch over.
On the other hand, if you have a new Samsung smart appliance, 2018 Samsung smart television, or the Samsung Connect Home WiFi system, support is telling people to use the Connect app for those and as many rules as you can, but to install and use the Classic app if there is missing functionality that you need.
On top of that, the iOS and android versions have different features.
And unless something has changed in the last day or so, you can’t use custom DTHs with the Connect app. You can use custom smartapps, but only if they are first installed through the Classic app. But features in Connect are changing rapidly, i don’t know if this paragraph is still true.
There’s also no way to synch the two sets of automations either through the apps or the IDE and to be honest it’s a mess right now.
Tagging @wb70 who has been following the issue in detail.