I have the SL1 and the 18-55mm STM. It's good, but I can't give you any opinion on which is better.. but I like the 18-55mm STM... I used to have a 18-55mm IS version and it SUCKED. I wanted to smash it. Comparitively, a world of difference. The zoom and focus rings are very smooth and video works well. Besides that, the 18-55mm is way cheaper leaving you cash to spend on a prime (40mm, 50mm, 85mm)!
If this isn't your first dslr, I don't know if you really want the 18-55mm. Investing in good quality glass is worth it in the long run. Just saying.