Catan Universe
I think Catan Universe is not easy to use unless you unlock at least the base game.
For those considering purchasing one of the apps:
I have both apps. Catan Classic is the old version and is no longer being updated. However, Catan Universe is notorious for being really buggy.
I believe Catan Universe has a larger user base and it's available via web browser.
If you are planning on buying one you may as well buy Catan Classic because if you have it you can unlock Catan Universe for free so for approximately the same price you can get both. (If you want Seafarers or C&K the same applies, buy them in Catan Classic and get them free in Catan Universe.)