I switched because I could not get over the fact that the Nexus S is a 6 month old phone. It was hard for me to give it up, for the reasons you just stated...but I could not resist the latest and greatest hardware on the 3D.
With my nexus I had issues with the radios like most ppl, my 4G was dead slow compared to the EVO, and my browser performance and game performance on the nexus did not compare to the 3D. And you can only tether the 3G connection, not 4G yet for some reason.
But the 3D is not bad when it comes to battery life in my opinion, and I feel like ppl will agree. Plus, once root is achieved, I will also have free wireless tethering, faster updated, and even better battery life.
I have been considering the same. I have the 3D, but I'm not sure I want to keep it. Enough little hitches make me reconsider my early adopter status on this phone.
I will wait until the Nexus S 4G update drops later this week and gauge the response to the radio fixes and other tweaks (maybe even 2.3.6?). Depending on that news, I may jump ship. It will be hard going "back in time" in many respects, so I don't know which way I'll go yet.