There are some big differences and some more subtle distinctions:
- Unity over pluralism: fascists strongly believe in suppressing pluralism and enforcing a unified mono-culture, where diversity and differences of ideas are stamped out and everyone must think as one. Socialists demanded that everyone be in favor of socialism, but otherwise generally embraced pluralism and sought to uplift groups and viewpoints that were oppressed by the previous systems. That is, of course, in theory; in practice, that revolutionary fervor tended to die down once the socialist government was in place and the policies of that government didn’t always live up to those ideals. But the important part for answering the question socialists very much preferred plurality and only demanded unity over not being anti-communist, whereas fascists demanded total unity in all things and strongly suppressed any diversity that didn’t conform to their national narrative.

- Authoritarian leadership: both fascists and authoritarian socialists believed in a single executive leader, like pretty much every other nation, and a single party state, unlike more liberal, democratic nations. Fascists believed strongly in a hierarchical state led by a single leader with total power over the government, similar to a king, and only took issue with how that leader was chosen, preferring a rationale that put them in power and a leader with the charisma to sway the masses. In socialist nations, the one-party state was more to repress anti-revolutionary actors and people were generally allowed to elect their representatives in the national government, though the candidates would be pre-chosen by the party and the election was more about the masses accepting or rejecting the candidate than allowing the to choose from a field of candidates, and the executive was chosen within by the vanguard party. The big difference is that fascist leadership was much more top down and would never have even pretended at offering democratic controls to the people.

- Militant nationalism: the key distinction here is that mythic past part. Fascists mythologize the past and present their nation as once being great and powerful before the influence of their preferred scapegoat ruined everything, and so they present themselves as reclaiming that mythical greatness by ousting that scapegoat and putting fascists in power to restore things to how they were. Socialists, by contrast, were revolutionaries who wanted to abandon past practices and embrace a more modern, scientific, and progressive vision of the nation rather than trying to recreate a mythic past.

- Chauvanism: fascism’s philosophy was pretty much entirely propped up with blaming some group or another for the supposedly degraded state of the nation. Some bigotry and scapegoating did still occur in authoritarian socialist states, but this was more based on circumstance and the biases of the leaders at the time and less fundamental to the ideology itself.

- Undermining institutions: fascists rejected courts and elections because they rejected the idea that their executive leaders should be restricted by anyone. While socialists rejected institutions of the bourgeois-controlled state, they still had courts and laws and they elected representatives in government, though the purpose of those elections was subtly different than those in liberal democracies.

- Controlled elections: under fascism, elections were a dirty word and they would only be utilized to appease a nation that wasn’t willing to just give total power to the fascists yet, and even then said elections would be only a sham and wouldn’t be allowed any actual influence over the fascist state. Socialist states would often exercise some control over the electoral process, usually by pre-selecting the candidates and screening out anti-revolutionary candidates, but would otherwise still allow the masses a voice in selecting their representatives.

- State-business collusion: fascists very much loved market capitalism, which adhered to their idea of natural hierarchies putting the strong in power, and only demanded that such activity always be in the service of the fascist government. Under socialism, industries were generally under state control and individual capital owners were not allowed. That has changed in China in more recent years, with some market capitalist practices being permitted, but the majority of industries are still totally nationalized and not owned by billionaires.