Depends on the game, what the High and Ultra presets entail and also how much value you put in certain visual aspects, but I'd say 95% of the time 1440p on High will look better. The difference between High and Ultra *usually* isn't that big at all, whereas the difference between 1080p and 1440p is fairly undeniable and shouldn't need a side-by-side to make the point.
EDIT: This is assuming you're talking native resolution(1440p monitor vs 1080p monitor) and not downsampling 1440p to 1080p. If you're talking about a 1080p display and what resolution to set, it becomes a far more debatable question. Image benefits of downsampling from 1440p to 1080p aren't massive, but *might* trump other effects somewhat. As an example, I'm currently running Rise of the Tomb Raider at 1440p with certain settings turned down to keep performance acceptable. I feel that the game's aliasing is quite bad at 1080p so the benefits of the extra AA that come from downsampling from 1440p benefit me more than cranking up shadows slightly or some near meaningless reflection and hair settings and whatnot. But I might not feel that way in another game, like with The Witcher 3, where the foliage draw distances and whatnot are more worth turning up(though I still never used Hairworks...).