Basically I looked at GDP growth rate per year and calculated the average over the last 10 presidential terms (which, not surprisingly to me, was evenly split between Democratic and Republican Presidents). I then made a graph correlating the two. Here are the details:
Obama avg GDP growth D 3.417
Bush Jr avg GDP growth R 4.220
Clinton avg GDP growth D 5.747
Bush Sr avg GDP growth R 5.475
Reagan avg GDP growth R 7.711
Carter avg GDP growth D 11.492
Ford avg GDP growth R 9.965
Nixon avg GDP growth R 8.755
Johnson avg GDP growth D 8.043
Kennedy avg GDP growth D 6.567
Democratic Average: 7.0532
Republican Average: 7.2252
We can see that the Republican party has a stronger, more positive effect on the economy.