Not to shoot holes in your logic, but the Nexus S was released May 8th 2011, while the HTC Evo 4G was released June 4th 2010.
A more apt comparison might be between the Nexus S and EVO 3D (June 24th 2011) or OG EVO and Nexus One (Jan 5 2010 release)
The Nexus S will likely see more software upgrades than the EVO 3D. The Nexus S already has ICS and is on the docket for Jelly bean, while HTC said that the EVO 3D will be upgraded to ICS, and the HTC EVO V 4G (rebranded 3D) has ICS. It hasn't rolled out yet AFAIK... I highly doubt the EVO 3D will see any further upgrades though.
The OG EVO has been retired for software upgrades, as well as the Nexus One. Evo retired at Android 2.3.3 with the Nexus One retiring at 2.3.6
I will say though, the Nexus line, seems to do a lot better on new operating systems than other phones. It seems like Google really takes their upgrade seriously and wants the phone to be able to access all the new features in the new OS, instead of just rolling out an upgrade to "please" the customers, like some OEMs have done.
I am a big fan of the Nexus line and will not go back to anything other than vanilla android. All that other nonsense just bothers me.