Trotsky was a purely "fundamentalist" idealist, while Stalin was a relatively realistic pragmatist.
Of course, in the basic Bolshevik theory, in fact, there is no essential difference between the two at the root, the difference is at the level of real operation.
In the early 1920s, when the proletarian revolution was relatively dark, the Soviet army was actually defeated by the Polish army and had to cede half of Belarus and Ukraine, and the Soviet Union's export of foreign revolutions continued to be frustrated.
Whether to continue the global revolution or to focus on domestic construction, Trotsky and Stalin's concepts were different.
For example, in the "continuation of the revolution", Trotsky always insisted on exporting revolution and advocated a world revolution; he regarded the proletariat of Britain, France, Germany, and other countries as the backbone of the proletarian revolution in the whole world, and he hoped to instigate the proletarian revolution in these old capitalist countries, so that he could seize the social wealth of the richest and most powerful countries in the world, thus better supporting the world revolution in Bushervik, and thus realizing the dictatorship of the proletariat in the whole world or at least in all Europe, if the revolution stopped in the Soviet Union. That is bound to retreat and even collapse in the face of the West, which has a huge advantage.
However, in Stalin's view, the soviet union's strength is still very weak, especially the capitalist forces in the West are far more powerful than the Soviet Union, and after defeating the Western interference, the top priority of the Soviet Union is to develop the economy, carry out industrialization as soon as possible, and rush to carry out a "global revolution" before the strength rises.
Moreover, Trotsky advocated the creation of a workers' autonomous planned economy, while Stalin wanted to build a top-down bureaucratic planned economy; Trotsky advocated the establishment of a proletarian multi-party system, while Stalin emphasized a one-party system.
To a large extent, Trotsky was more radical, and Stalin was relatively more pragmatic. This is actually a bit like the difference in ideas of Castro and Che Guevara, except that the old Ka and Che are just different in concept but have a good relationship, and Stalin and Trotsky are purely different in concept and fiercely competitive.
Of course, even if Trotsky had reigned in the 1930s, his line would not have been much different from Stalin's, except that he might have done a poor job in economic and state-building, but he would have been more ferocious than Stalin in suppressing the opposition; after all, while Lenin was still alive and Stalin was working as chief of staff, Trotsky had already executed on a large scale anyone he felt was unreliable, including the White Army and the Red Army, when Lenin was still alive and Stalin was working as a chief of staff.
Finally, talking about the competition between Trotsky and Stalin, in fact, in terms of seniority, as a senatorial figure, Trotsky is much more prestigious than Stalin in politics, it can be said that the two are not on a horizontal line, but Trotsky is as radical as ever, he has repeatedly clashed with Lenin, and he has constantly contradicted the top level of the Party, which has offended many people, causing most of the high-level people to exclude him collectively, while Stalin has always been relatively low-key, slowly entrenching and uniting others little by little, and finally overthrowing Trotsky. Until his expulsion from the Party and his expulsion from the Soviet Union, until Trotsky's assassination in 1940.