As some of the other posters were trying to say is the two devices may both be Android devices, but they tend to act two different ways.
The Nexus is Google's flagship phone, with the "pure Google experience" built in. That means that it is what is referred to as a vanilla Android OS with no carrier software or manufacturer interfaces. The Nexus is also supported for updates from Google itself. The Nexus is on ICS 4.0.4 now, i believe, and will be getting 4.1 jelly bean in the near future.
As for the Galaxy SII, even though it is around 8 months old, it is still a capable device. It does run Samsung's TouchWiz interface and has carrier apps installed. It did just receive the 4.0.3 ICS upgrade recently, showing that the device still has some value. I have one that i don't use any more due to i upgraded to the Galaxy Note, as i like the 5" screen size. If not for that, the Galaxy sII would still be in my pocket.
As mentioned, the Galaxy SIII was pre sold and customers are now getting them in the mail. Carriers are saying that if you didn't pre order one, you will have to wait at least a week or more to find one in the stores, as well as pay $300 for one on contract.
If you are on a budget, you can find the Nexus or SII for $100 or lower on contract. Last i looked, at&t had the GSII for free and i have seen the Nexus for around $50 on sale.
Sent from my SAMSUNG-SGH-I717 using Android Central Forums