I don't know much about the Legacy Sith, so I'll leave this with the fleets:
1) With the old tech taking on the new tech: Legacy wins by miles. While tactics and strategy are extremely useful, sometimes raw power and numbers simply can't be stopped with brilliant plans. Send one hundred men with rusted revolvers against one hundred men with fully automatic weapons, artillery, and the best body armor made in an open/clear battlefield, and I don't care how brilliant a tactician the leader of the first group may be. In flat out open warfare, they're going to get crushed by sheer power.
2) With the same tech? The KOTOR era Sith have an advantage. With the same tech, and Revan's tactical ability (Assuming, of course, that the Sith aren't going to be on a complete betrayal frenzy with the war going on), the KOTOR Sith will probably land a victory.