>>3337658Prime lenses have the advantage of often being sharper and they allow in more light (which is what the f-stop number means) which means superior low light performance. If a lens f-stop is below f2 then it's going to allow in a lot of light and also give you the creamy background effect through superior subject isolation. See pic related.
The kit lens tossed in the box usually has an f-stop of 3.5-5.6 which is usable outdoors but not fantastic in low light. So the first logical upgrade is to buy a prime like a 50mm f1.8 to gain the low light performance and depth of field advantages.
In terms of Canon vs Sony, the Canon will be more bulky and heavy than the Sony. The advantage of mirrorless is there's no mirror box inside the camera meaning they can make the cameras slim and compact. There are other advantages too but I'm sure you can find those with a Google search.
