A recent discussion in a DPReview.com forum thread highlighted the differences between these two ranges of Sony ultra-compact cameras, and I found it very useful for those who are similarly curious.
I have summarized the posts in that thread below.
The W series are classified as compacts, while the T series are smaller by design, hence they are called sub-compacts and are more pocketable. Miniaturization has resulted in the T series being the more expensive line of cameras. The T series are extremely thin by comparison, and are fittingly known as “lifestyle” cameras too.
Since the T series are smaller, their lens do not extend when zooming in (Sony calls it a “folded” lens)
Camera controls and features
The W series has more manual controls, giving more options to the user to set up the camera. The LCD display on the DSC-T50 doubles up as a touch screen user interface.
One strong functionality available in the T series is image stabilization in the lens. This helps with minimizing the incidence of blurry pictures from shaky hands. “R2D2” shares a magnificent set of image samples from his Sony DSC-T9. It is regrettable that the older W cameras up until the DSC-W100 don’t come with any image stabilization, although the newer models all do come with optical stabilization.
The W series come with an optical viewfinder, great for those who prefer to compose their pictures with the camera to their eye. The T series have no viewfinders — all composition is done via the live image on the rear LCD screen.
The T series are also known for their superb “Super Macro” feature, allowing you to shoot a subject as near as 1 cm away. The W series, on the other hand, only allow you to go as near as 2cm.
It is the general consensus that the built-in flash on the W models are more powerful, an important factor to consider if you take a lot of photographs indoors with flash.
The aperture in the W line of cameras can open as wide as f/2.8, compared with f/3.5 of the T series. The larger aperture in the W series allows for better low-light photography without the use of flash. The T series have a wider aperture at the telephoto end though.