Ever since both of them came out, Hell's Kitchen has been compared many times to Shokugeki no Souma. Personally, I think they are two completely different manga within the genre of cooking. If I were to sum up the major difference between Hell's Kitchen and Shokugeki no Souma, I'd say one is more about ideology while the other is more about impact.
Shokugeki no Souma is what I would call an impact manga. Like most manga today, Shokugeki no Souma focuses more on the competitive nature of the story. When you read Shokugeki no Souma, you care more about how strong Souma is and how much stronger he will grow as the story progressed. Hell's Kitchen on the other hand, focuses more on ideology. Like the idea that being nothing means you can become anything and the clash between science and art.
If you want to compare how realistic those two are, I'd say they are about the same. If you do a little research, you'll know that the techniques used in Hell's Kitchen is very hard to do while the techniques used in Shokugeki no Souma is very doable. However, people are more likely to have an orgasm eating Hell's Kitchen's food compared to Shokugeki no Souma's food due to the level of difficulty in techniques.
Regarding as to why Shokugeki no Souma got an anime, it's because it is more popular. The reason why it is so popular is because of the art.
If I were to choose, I'd say I like Hell's Kitchen more than Shokugeki no Souma. I'm a person who care less about the art and more about the idea of story. If you look at Shokugeki no Souma, you can tell that it relies a lot on the art to draw in the reader. Hell's Kitchen on the other hand, relies a lot on creativity in the plot and research in very unique cooking techniques to draw in the reader. It just depend on what you prefer.