ETH's success is 90% due to smart contracts and unless Ardor doesn't implement them one way or the other I don't even see it as a competitor.
The competition emerges as both can be used as a platform for ICOs and other kinds of crowdfunding, and also as a platform for "private currencies". Ethereum isn't really used much for its advanced smart contract functions. And Ardor's child chains have the advantage that no ARDR transaction fees would have to be paid for token transactions if a bundler is available.
As far as I know, this advantage is unique. Only in "empty blockchains" of Bitcoin-based coins, where zero-fee transactions are possible because of the low usage, this disadvantage does not exist, but empty blockchains aren't a sustainable model for a successful currency ...
And don't forget that many use cases that have gone Ethereum due to market effect could have perfectly used Nxt.
Associating smart contracts with Ethereum and not with Nxt/Ardor, in this sense, is a fallacy. A smart contract allows for self-enforcing rules in a blockchain, and the Nxt architecture provides modules that allow for self-regulating contracts (specially the whole phased transactions API) and other modules that work as a template for frequent uses in Ethereum, like asset/currency issuance and trade.
Turing completeness in Ethereum surely does allow in theory for virtually limitless custom tools, that is undisputed. But the idea behind Nxt/Ardor is modularity, which can simplify implementation of a lot of common cases, even if theoretically limited in scope so far. Enter the scalability provided with Ardor, which in the case of Ethereum (plasma, sharding...) still has a long way to go, and it is quite clear that, if we all put our fanboy hats aside, Ardor can and will be a competing technology.
Fair points that I didn't think of.
However, if you really wanna go toe-to-toe with the big guy (ETH), the ease of use should be comparable. People these days hate desktop wallets for some reason and the NXT one isn't really the most casual friendly or good looking one. On the other hand, nobody in their right mind runs an Ethereum full node at home either. This is why MEW is the de facto official ETH wallet. If Ardor/Jelurida offered a web wallet like MEW or a highly polished desktop wallet I believe this would help immensely.