From Salon's How the World Works.
I guess my own opinion (and not just on immigration but on everything) is that expansive and incusionary are better than restrictive and exclusionary.
If I were thinking solely as an economist, then the more information generated by more people making small decisions the better the value of that information is and the more competive the market is. So while we are all hemming and hawing about those immigrants using up resources- let's not forget that they don't live in a bubble. They buy food and cars and housing and clothes and, and, and. They pay sales tax. They pay income tax and social security (that goes into the system under fake social security numbers so there is a pool of money contributed by immigrants- especially into social security-) for which no tax return or benefit ever has to be paid out. We also need them to keep the age of the population lower. The older the general population is, the more a country's economy stagnates (go see Germany or any other european country with falling birthrates).
If markets are free, then labor should be as unrestricted as goods are when it comes to flowing across borders. Though if markets were actually free (no more crop subsidies!) then labor would get less benefit from moving across borders because they would have a chance at moving up in their own country.