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.