Saying that the Battle of the Marne caused Hitler is like saying that The Battle of Tsushima directly caused the October Revolution: German defeat doesn't neccessarily mean the rise of Hitler. The Weimar Republic could have survived and remained as a democracy, just as the Tsarist regime could have ground on through its reforms had it not been for the First World War. (But it could also have remained as a nasty, oppressive, but not USSR level of nastiness, state. Nicky was not a great reformer.) *sighs*
Still, whatever the AH speculation, the Germans, even if they had captured Stalingrad (not beyond the realms of possibility), could never have subdued the USSR, whereas if the German Army has taken Paris, the Great War would have came to a rapid Central Powers victory, which would have resulted in a radically different world today.
EDIT: And in terms of actual fighting, the Oktober Revolution was frankly a bit rubbish. A bunch of hilariously incompetent Bolsheviks supported by a cruiser with dud shells, led by Commissars who couldn't tell broken down artillery from clean pieces and lost their signal lanterns (source: "Young Stalin") vs the Provisional Government defended by, as a group of deserting Cossacks called it, a "bunch of Jews and wenches": a handful of Cossacks, a Women's Batallion (who posed for photographs on their barricade), and some military cadets who left the Winter palace for supper, never to be seen again. More people died in the making of Eisenstein's film than in the actual engagement (although several of the Women's Batallion ended up being raped, and the entire old Tsarist wine cellar was looted, despite repeated attempts by Lenin to stop the massive orgy of boozing that raged on, with wine literally running through the streets.) It wasn't, as revolutions go, a very good omen for the government that would follow...