You accuse Robert Service of bias, and then recommend Trotsky's version of history is as a less biased alternative -- you're having a laugh! There could scarcely be a more biased, not to mention self-serving, source than Trotsky.
As for the question of whether Lenin's leanings were dictatorial or democratic, the answer is clear from Lenin's own writings, as well as his actions. Lenin did not merely have a "different understanding" of democracy; he saw democracy described by Marx, and rejected it. Let us recall that Marx described "dictatorship of the proletariat" by reference to the system of representation by universal suffrage that operated in the Paris Commune, in which all the councillors were workers, paid workers' wages, and subject to instant recall. Lenin decided that this system would not suit Russia, and put forward his own alternative, according to which the proletariat needed to be led by a "vanguard" (by happenstance, self-selected) of professional "revolutionaries" (which, as luck would have it, turned out to be himself and his Bolshevik cronies). So much for Lenin the democrat.
Robert Service is right to be anti-Communist, and his being so does not necessarily make his history biased. Would you say that a biography of Mussolini could only be unbiased if it were written by a Fascist?