I rated each team using the formula 1.7*OBP + SLG. I did that for their opponents as well. Then I found their differential for the 1st half of the season, the 2nd, Sept/Oct and the post season (each series was weighted by the number of games). The table below has the results (clicking on it will enlarge it).
The Rangers were clearly the superior team in the 1st half, with a .064 differential vs. the Giants .038. But in the 2nd half the Giants pulled slightly ahead, .069 vs. .058. That is largely the result of incredibly great pitching. They held their opponents to a .297 OBP and a .373 SLG. But in Sept/Oct those numbers were .251 & .292 (their ERA was 1.91, over 29 games!). They almost kept it up in the playoffs, so far with .274 & .298.
In Sept/Oct, the Giants have a huge edge in the differential, .193 to .052. The Rangers have the big lead so far in the post season, .200 to .069. So no clear winner. It would have been nice if one team had a bigger differential in all cases. I think the Rangers will win, however, because I just don't see how any team can keep up the super human pitching the Giants have displayed. One other reason is that the Rangers have probably faced tougher competition, based on the fact that the AL once again won the majority of inter-league games.