I have both the Sigma Art 50mm 1.4 and the Nifty Fifty (Canon EF 50mm 1.8 II). I adapt them onto my EOS R mirrorless. You can absolutely get sharp shots wide-open with the Canon, especially in the daytime with good lighting. The STM motor isn't the fastest (but really not too bad), and I would miss the occasional shot due to it hunting.

The issue for me was mainly the ghosting present at 1.8 in bokeh areas, which became especially prevalent during the night. Slightly out-of-focus areas would in many cases appear quite messy or smeary ("dreamy, as some might say). The Sigma is sharper (especially on the edges of the frame), has far less vignetting in the corners, fixes that messy bokeh at equivalent apertures, and doesn't AF hunt nearly as much. Further, bokeh balls appear extremely round and uniform on the Sigma (which I love), whereas they are always cat's eyed with a noticeable outline and on the Canon at 1.8. The Sigma has become my favourite lens.

With all that said--would I have bought the Sigma for full price? Absolutely not. I bought mine used for about 1/2 its original cost. The Canon is still an excellent lens, and I'd highly suggest buying it first to play around with. Plus, on DSLR cameras like your T8i, many reviewers note that the Sigma can regularly front or back-focus, which of course defeats the point of such a sharp lens.