Since you've said you want something manageable in the city and also stand out from the sea of commuters, we'd recommend taking a look at the Honda CB350 and the Triumph Speed T4. 

Both have upright ergonomics, good ride comfort and have a well-balanced chassis which ticks all the boxes you're looking for. Of the two, the Speed T4 will offer more performance than the Honda, and both are similarly fuel efficient. 

Also, Bajaj will be downsizing all the Triumph 400s to 350cc to take advantage of the new GST 2.0 regime, and these bikes should be released by the first half of the year. Once this move is complete, expect prices across the board to be quite a bit lower, and if you are shopping in this space, it would be prudent to wait a few more months. 

The KTM 250 Duke you are considering makes for a great one-bike garage to handle city and highway duties both. It has a punchy - yet not overwhelming - amount of grunt, engaging handling and will also return good fuel economy. 

Coming to the Triumph Speed T4, depending on how you ride, you should see decent fuel economy even inside city traffic. Since the bike is made by Bajaj, it shouldn't be too expensive to maintain. The advantage of the Speed T4 over the Duke will be its more effortless performance and more comfortable seating and suspension. 

And since you are shopping in this space, the TVS Apache RTR 310 is also worth taking a look at. The RTR 310 is a feature-rich machine with good performance and handling. Also, we will recommend you get the variant with the quickshifter as it is still priced sensibly, and this feature will enhance the riding experience. 

Since these are different machines in the riding experience they deliver, we'd recommend a test ride of all three before you decide. 

If you want something with a bit of sporting intent in the way it rides, the Triumph Speed 400 makes for a good purchase. It is a relaxed city commuter with good suspension, a smooth and grunty engine, as well as an agile demeanour. If you want a more mellow riding experience but want a Triumph, the Speed T4 makes a good case for itself. 

If you want bikes with a more old-school riding experience, then the Honda CB350 models and the Royal Enfield Bullet / Classic / Hunter 350 models deserve your attention. 

All bikes here have been out for multiple years now and have largely proven themselves to be reliable machines. 

A test ride of these bikes will help you decide which one suits you the best. 

The Triumph Speed T4 will make for a better all-rounder because of its larger and more flexible 398cc engine. It is perfectly usable inside the city, and on the highway, you can cruise at early triple-digit speeds without breaking a sweat. 

If you want similar looks but more performance and better componentry, the Triumph Speed 400 makes a good case for itself. 

The Yamaha XSR155 is a much smaller - and lighter - bike that will have better handling and fuel economy, but will feel a little out of its element on the highway. 

You should also look at the Royal Enfield Bullet / Classic / Hunter 350 models (depending on your budget and design preferences). They are smooth, charming and comfortable machines that can be used both inside the city and on the highway. For similar reasons, the Honda CB350 trio also warrant serious consideration. 

A test ride of all of these, as well as anything else you're considering, will help you decide which one is the best fit for you.