I used the Nexus S for two weeks, traded it in within my 30 days for Evo 3d and have had it since the day it came out, here's my opinions on the two phones:
Nexus S-- Much nicer build quality, much nicer screen (yes slightly lower resolution, but AMOLED just looks sooo much nicer compared to qHD). Very smooth for using basic integrated apps, but single-core 1Ghz CPU chugs pretty often if you try to get into "hardcore user" type stuff. Not too very difficult to root/flash, I ended up with both vanilla AOSP and CM7 on mine, overclocked to about 1.3Ghz (note, still chugged noticeably on games and other things).
Overall, much better out-of-the-box experience, if you just want a great phone to use as-is and do things similar to what LegendarySoul said he wanted to do, then DEFINITELY get this phone.
Evo 3D-- Lower build quality, I remember being disappointed with the out-of-the-box impression. The back cover is a cool rubberized texture, but the rocker/power keys are covered with it too, making them very annoying to press compared to the Nexus S (this was the hardest thing about the switch from Nexus S to Evo 3D) qHD looks nice, I switched everything over to Serif font just because it had the resolution to show it off and it looked really cool. Not anywhere near as vibrant as AMOLED though. 3d screen and camera are pretty cool, but if you buy this phone for that express purpose, you'll probably be disappointed at the options (not many movies/games, and can't use the GUI in 3d). However, if you just want this phone for the specs then the 3d is a very cool side-feature. Not rooted yet so can't say anything about flashing or customizing. Bloatware boarders on absurd. Now to the good part The hardware on this sucker is just outstanding. Dual-core 1.2 GHz means that even when 3d gaming or doing really hardcore stuff, I haven't even seen it so much as stutter.
If you're a hardcore user and really like to tinker with, and get the absolute max out of your phone, DEFINITELY go with the Evo 3d. Just messing with them side by side would make you think Nexus S is better, (HINT: This is why reviewers don't like the Evo 3d very much! They only test it for a day or so and don't actually use them both for 2 weeks like I did ) but once you get down to the nitty gritty the Evo 3d blows it out of the water.
Hope someone finds this useful, if not at least it was fun to write