The first is that the argument from moderation is a bit lazy. It is true that a lot of times, moderation is better than extremism. But it's not always true. Barry Goldwater was kind of an asshole, and but I think he was right when he said that (to paraphrase a little) extremism in the defense of liberty is no vice, and moderation in the pursuit of justice is no virtue. If we accept the political equality of democracy as one extreme, and the absolute tyranny of a single individual as the other extreme, is the moderation of of submission to an oligarchy therefore superior? If slavery is one extreme, and liberty another, where's the moderate position? A little slavery? Just that "X is moderate relative to Y and Z" is not by itself a sufficient argument for X.
The second issue is that the history of the distinction between "socialism" and "communism" is complicated. We can divide the ideological history of modern communism and socialism broadly into four periods:
- Utopian socialism (pre-Marx)
- Marxian communism
- Early revolutionary communism (pre-1917)
- Communism of the parties
Marx and Engels both criticized utopian socialism. One important difference between utopian socialism and Marxian communism is that the former typically is philosophically idealistic (the material realization of an independently-existing ideal form) and the latter is materialistic (development by the interaction of fully physical elements). Engels draws the distinction at length in Socialism, Utopian and Scientific, 1892.
In Marx and Engels, or at least the translations I've read, socialism is either identically synonymous with communism, or socialism refers to an "early" stage of communism, when we still have to have social institutions to manage production; communism refers to a later stage, when the means of production have become sufficiently advanced that people can just work at whatever pleases them, without having to worry about the social utility of their labor. Lenin usually follows the latter distinction.
In the early revolutionary phase, when actual communists were trying to gain state power, a sharp distinction developed: between communists and "democratic socialists." The fundamental character of the latter was a desire to preserve and work within the bourgeois representative "democratic" (i.e. republican) system. Marx touches on the distinction in Critique of the Gotha Programme, but Lenin has the most thorough treatment in What is to Be Done? and The State and Revolution.
Finally, the Communist Party of the Soviet Union and the Chinese Communist Party came to define (or legitimatize the de facto definition) of communism as rule by the meritocratic national Communist Party, hence the modern term "Communism of the Parties."
The distinction between communism and socialism is therefore kind of equivocal. If you look at the distinction drawn in the revolutionary phase, "socialism" means the preservation of representative democracy, and "communism" means the rejection of representative republicanism as fundamentally bourgeois, in favor of some other unspecified system. On the other hand, one might use a distinction based on Communism of the Parties, where "communism" means rule by the national Communist Party, and "socialism" means rejection of party rule in favor of some other alternative.
I personally reject both bourgeois republicanism and meritocratic rule of the Party, in favor more directly and thoroughly democratic political systems, such as the systems briefly implemented by the Paris Commune, of which Marx wrote approvingly in The Civil War in France. If people want to call themselves "socialists," and they mean a rejection of both rule of the party and bourgeois republicanism, then I'll call them comrades without quibbling over labels.