I'd like to have your opinion on this
We know that Shinichi Kudo is a genius
This mainly has to do with his upbringing, both his parents being geniuses themselves.
His mother is a natural-born actress, while his father has posesses above average-intelligence.
Moreover, the Kudos seem to be quite wealthy. This allows Shinichi to travel around the world, pick up interesting skills and experience things few people have.
Oh, and he has an immense library, which is likely where Shinichi got most of his knowledge from. We do know he likes to read a lot.
Then we got to see a glimpse Shinichi's childhood. He's not even 5 yet, but we are shown that he already posesses a precocious intellect. He is unusually observant for his age, and he can even read Sherlock Holmes stories on his own. So yeah, Shinichi really is a genius.
Then there is the fact that, at the age of 17, he is already an accomplished detective.
I repeat, he is but a mere adolescent, but his skills surpass those of professional investigators.
To compare, let's take the example of the DDS class from the Tantei Gakuen Q manga. Like Shinichi, they have solved tons of criminal cases already.
However, they are still STUDENTS. Which means they are still training. They are still learning to devellop their skills as detectives.
But as I said, we never see Shinichi do that. He has a yearning for mystery novels, and is father took him to crime scenes since he was little. Adding to all that his genius intellect, vast culture and keen senses, this is what makes Shinichi an accomplished detective.
Now, going into the subject, let's talk about Heiji
His father is a high-ranking policeman, and his mother seems to be a housewife
He has a rather mundane upbringing. But compared to Shinichi, Heiji is a lot more hot blooded
His father is quite strong and strict, and his mother is described as very supportive. Plus she practices martial arts.
These traits seem to have be bestowed on their son.
He is very hard working, brave, impulsive and probably practices kendo to vent out all this energy he has
But the thing is, Heiji doesn't strike me as a genius like Shinichi.
So then, where does his detective skills come from ?
As I said, Heiji is very hard working (contrary to a genius), and he is also very determined
I don't picture him enjoying reading a lot of books like Shinichi would. But if he wants to do something, he would surely do it.
Heiji has to double efforts in order to be a detective, and to be able to gain all the knowledge he has, it is likely to he had to force himself into reading books
According to me, the fact that he wants to be a detective stems from three things
1) His father is a cop. A high-ranking one in fact. He admires his father and since he has grown into this environment, it is natural that he would be interested in criminal investigations
2) He is hot blooded as already stated. Just like kendo, catching criminals permits him to use all his energy and experience a satisfying feeling when he has succeeded in stopping the bas guys.
3) He is very competitive, so of course when he does something he wants to be the best in the world. Which is why he desires to be the best detective in the world
Now, contrary to Shinichi, Heiji is much more prone to errors (we see this in his first appearance and also teh Detective Koshien case)
Moreover, he is a little bit dense when is isn't in full concentration mode (almost always calling Conan 'Kudo' in front of others)
He is also reckless, he gets hurt more than anyone else in the manga. He hardly thinks before he decides to act. Conan is more of a planner or strategist. Heiji would just beat up anyone who annoys him.
And that is why I think that Heiji is not a genius detective, but just a hard working teenage boy
Technically, he deserves more praise than Shinichi, whose skills were practically handed to him from his birth
Heiji is just a level below that of genius Shinichi thanks to sheer determination, and that is an accomplishment