Where the Lumia 735 has consistently exceeded my expectations in ways that are delightful, the Lumia 830 has instead not met some of my admittedly unreasonable expectations. The model numbers suggest a natural step up in quality, features and performance from the 735 to the 830, but that's not the case, and in some important ways the Lumia 735 outperforms—or is otherwise better than—the 830. This is confusing, and while it's fair to blame Microsoft for not more obviously differentiating the two in the way I expected—e.g. that the Lumia 830 would be generally "better" than the 735—what we're left with is the current situation. And it goes something like this.
The Lumia 735 and 830 are essentially comparable, technically and functionally, and from a performance perspective. What really differentiates these handsets is a couple of unique features for each, the form factor/body style, and of course the carrier availability that will trump any need to compare them in the first place. What's up in the air is the pricing—still, and that blows my mind—but based on European pricing, the Lumia 830 is more expensive than the Lumia 735 and, I have to be honest here, I'm not sure at all that that is justified. (I expect a no-contract 735 to cost about $300 in the US.)
The killer here, for the Lumia 830, then, is that the things that make the Lumia 735 special will be more generally appealing to people than the things that make the Lumia 830 special. So aside from its lower price, the 735 is indeed the "better" phone. For me, personally, the weird bit is that this is true for me as well. Despite my love of high-end Lumia cameras, I'm sorry, Microsoft, but the Lumia 830 is no Lumia 1020. It's no 1520, Icon or 930 either. It's just not in the same ballpark.