The EF-S 10-18mm can be a very useful addition to your T3i kit. On a crop-sensor body, it gives you an ultra-wide view that your 18-55mm can’t, which makes it well suited to broad landscapes, sunrises/sunsets, dramatic skies, and fitting a large portion of the night sky into the frame.

For stars and other low-light night work, though, aperture matters a lot. The 10-18mm is wide, but it isn’t very fast, so a lens with a wider maximum aperture can make night shooting easier and help keep ISO lower. A fast standard zoom or prime was suggested for that reason.

So the decision depends on what gap you want to fill:

• Keep the 10-18mm if you want wider landscapes and sky shots.
• Consider a faster lens if your priority is low-light or night photography.

Since this is also about shooting style, the best advice is to test it in the kinds of scenes you actually shoot. If it solves a problem your current lenses don’t, it’s worth keeping. Renting before buying another lens is also a smart way to decide.