I have a Canon T6i and I am looking to buy a new lens, I am mostly interested in product photography, sharpness and image quality is super important to me. I am currently using a kit lens which is Canon EFS 18-135mm and I am thinking about buying Sigma 24-105mm f/4 DG OS HSM Art and it's very expensive so I need to get some advice from an expert before buying it. Is it worth it or not? Am I ganna see a big difference in the quality? What is your advice to me?
If you're not ready to assess the value of a lens, it seems to me that you're not ready to buy one.
If you want to do product photography, you need two things: a reasonably long lens, and a tripod. The Sigma might be okay above 85mm, but I'd suggest a macro lens at 90mm or more. You want longer to mitigate any geometric anomalies you'd get with a shorter focal length.
If you're still not sure, rent one if you can. Not knowing where you're located (fill out your profile?) I can only suggest.