In my opinion it sounds like you're frustrated a bit. Frustration most of the time leads to wrong decisions.
Now, to answer your question: Nexus S 4G or Evo 3D?
these two phones are almost incomparble to each other. They both offer features that the other one doesn't offer.
Nexus offers a Super AMOLED screen, thin design, the most recent up-to-day version of Android, and a couple of other things.
With the Evo 3D you get the support from HTC. In my opinion, HTC has the best looking software out of all the manufacturers outhere. The 3D camera might suprise u or might dissapoint you, so for that I would suggest you physically touch the phon and play with the camera a bit. The first time I saw it I didn't like it, but I went back later on another ocassion and I was really interested on the way the pictures looked.
I will still choose the Nexus because of the screen and Android.
On another note, the issues with the battery you're referring to, I was having the same issue with SFR 1.2, but after switching to EG22 that bug was gone and I have seen an increase of battery life compared to stock EC05 and custom ROMs based of EC05.
All in all, I think you have a great device with you already. I would try EG22 first, using Titanium Backup pro to freeze or get rid of anything you don't use on that rom, and maybe using SetCPU to overclock for the lag issue or underclock for saving more precious battery life. I use Overclock widget as it allows me to define the frequencies I want my phone to run at. Even underclocking my Epic I see no signs of lag. But in the end, I wish you good luck in your endevour as you try to decide whether to upgrade your phone or try different things with the Epic, as each phone is different, but there's only one perfect phone for each one of us.