This will be London's dreby and in a derby there everything is possible, this handicap on Arsenal represents value because Arsenal are better team than Chelsea and they are playing great football with great confidence on the road this year. In a derby like this the home advantage is not so important as for other games, for ex. remember the last Chelsea's game against Tottenham 4-4 or Tottenham vs Arsenal 1-3 or Arsenal-Tottenham 2-1, both games ended in results which haven't represented what happened on the pitch as Spurs have created the same number of chances but they have been simply unlucky. Remember the game Chelsea vs Liverpool when the home side didn't showed a better football than Benitez's side and the handicap before the match was -0.25 even more on -0.5 on Chelsea. Arsenal and Man. United are the only teams in Premier League which a really playing great football and IMO Arsenal are better than Chelsea and as this game is a derby there will be more chances for a draw like in every derby. About motivatin factor there are no doubts about Arsene Wanger's team after their bad results in a row.