Obviously it makes a difference
in general who is president. The point being argued is that the particular two candidates we got aren't likely to be that much different.
Back in 2008 I certainly thought and hoped Obama would be a big change from Bush, but it seemed like
nothing changed. He continued or expanded Bush's policies of warrantless wiretapping, the Bush tax cuts, increasing executive powers (in often-unconstitutional ways, such as the war in Libya remaining unauthorized by congress), continuing the war on drugs, etc. The Wall Street "reform" bill was the smallest imaginable response to the horrendous behavior of the financial firms. Troop deployments were not immediately decreased but merely shifted around. There was that new "insurance care" bill, but if Romney had won his presidential bid in 2008, isn't it possible, even likely, that we would have ended up with something similar from him?
You mention Iraq--we hope neither candidate would start another war the way Bush did, but do we really know they wouldn't? And Obama didn't really bring peace to Iraq, did he?