I honestly thinks the Nexus S is the best phone out right now.
The top 3 phones are probably the iPhone 4, GS2, and Nexus S.
They all have the best screens - Retina/SAmoled kick every LCD on HTC's phones a**, and most other devices have some major flaw.
Compared to the iPhone, Android may be the better choice, and the larger screen is much better.
Compared to GS2, GS2 may be faster BUT I honestly think it is *too* large, and the OS is Samsung-based, I love Nexus S' vanilla android, plus it is going to receive an update soon, as it is straight from Google.
I really think the Nexus S has all the packages - it is not really missing anything!
That said, you can wait for the N-Prime but from what I see, it is only going to have soft-core advantages. These would include the typical small camera upgrade, a bit of screen resolution, 1GHz to 1.5GHz, maybe single to dual core, a bit of RAM, and that's about it. Both will run the same OS, and from what I read - the Prime won't have soft buttons anymore - a bummer.