The first difference between a Smart TV and a normal TV is that, the smart one can access the internet through its Wi-Fi system, while the standard or normal one cannot. A few Smart TVs currently are even having the facility of running apps that has been built into the TV.
But, if you are talking about performance, then it would involve a lot of factors which have got nothing to do with the picture quality. In other words, the picture quality doesn't have any link with a TV being smart or ordinary. What matters, is the display type, for example, a smart TV can either have an LED display or have an LCD display. Currently the HD display mode that are said to be displaying best quality pictures. Again, within HD display, there are two varieties, normal HD and Full HD.
So if you buy an LED TV with any kind of HD Display, you won't have to worry about the picture quality. Buying a smart TV on other hand is just getting a few more facilities added to it, which we didn't have in the standard ones.