For a first time user, would you recommend the nexus s or atrix?
Both are great phones, and your choice must be based upon your own preferences and needs.
The 2 features of the Nexus S which are most often called out as negatives are:
Lack of SD card facility. You are limited to the 16 gigs of internal storage.
Lack of an indicator LED, to tell you about email and other alerts
Then there are a bunch of others, which you can easily search reviews for.
On the other hand, a few plusses for the Nexus S:
A superb display
Frequent and direct software updates direct from Google/Android
The phone is unlocked and capable of all manner of the dev's mods. And you get the "pure Android experience".
And so on...
For me, the lack of the SD card facility is not an issue at all. I do frequent file exchanges with my desktop machine, so I do not need 32 gigs of removable storage for my phone.
I love the fact that you get the latest and best software direct from Google/Android. You are never dependent upon the device maker and the cell phone provider for updates.
I have no need for Motoblur or any of these other manufacturer's "skins".
So, the Nexus S has many things which I like. Your needs may differ.
For the Atrix, there is the incredible speed. Many folks like the MotoBlur skin. They like the gigantic removable storage facility. And there is the hope and belief that Andoid 2.3 will come, someday, which promises to unleash still more power in the Atrix processor, among other things.