I think Afghanistan will be a harder job then Iraq. I think the upside to being successful in Iraq is much higher in Iraq than in Afghanistan. In terms of strictly defeating global terrorism, I think Iraq has more potential than Afghanistan. Despite this, I think Afghanistan is worth the treasure and lives we have spent and are continuing to spend.
The major downside of going into Afghanistan & Iraq is, we will probably be quite successful, therefore many might see this "type of solution" as applicible to other regions in the world- and it probably won't be. One reason it won't be "similar" is that even if we had the exact same situation in Iraq, Saddam II is unlikely to make the same mistakes: he knows the US isn't merely bluffing, he knows that the US forces are much more competent than his forces, and etc.
If Saddam had fully cooperated with the UN inspectors, we would have been forced to stand down, and leave the Iraqis with their monster dictator. Fully cooperating with UN inspectors would have eroded Saddam's power, but would have been possible for him to continuing robbing and murdering the Iraqi people, and once America focus had shifted to other matters, he could have regained his former glory. Saddam had endless warnings, next time we might not be willing to give such a ruthless thug, so many chances- and that would be bad, we should always try as much diplomatic solutions as possible.