Each $1 the U.S. government doles out to the unemployed results in $1.61 in economic stimulus. Welfare/food stamps are even better: $1.74. The Bush tax cuts? $0.32. The way this works is that when an unemployed person or welfare recipient gets their check, they spend what they're given. The people they pay for goods and services then spend the money they earn so the money goes right back into the economy a 2nd time. Thus each $1 can result in more than $1 in stimulus. Much of the Bush Tax cuts (and that's all of them, not just the tax cuts for the wealthy) go into people's pockets without additional spending for the economy, because most of the tax benefit goes to higher income earners - either the wealthy, who get the lions share, or the middle class who is still working, who is currently focused on reducing family debt levels and saving more for the future. So just $0.32 out of each $1 goes back into the economy. It's still stimulative, but at less than 1/5th the stimulative effect of extending unemployment benefits.
This chart shows how much of the tax cuts go to each income group.
The most stimulative of these tax savings are in the lowest income brackets, probably those <$100,000 or so. Those are the people most likely to put the money directly back into the economy through increased purchasing power. Everyone else is much more likely to save money or invest it, which while theoretically a force for the economy, is much less so now that investment firms traffic so heavily in the derivatives markets.
And as you can see, the Republican plan, which is the plan that has just been agreed upon in yesterday's compromise between the Obama Administration and Congressional Republicans, gives by far the largest tax break to the wealthy who will be the least stimulative - they're not only least likely to spend their money, they're also the most likely to actively seek out complex securities for investment.