I want to buy my first bike. I currently have an Ather scooter and a Splendor, which I ride very little. I want a retro-looking bike with good or decent mileage that is suitable for city riding, highway use, and occasional touring. I am considering the Triumph Speed T4 or the Yamaha XSR155, and I am open to other recommendations.

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.