Ok, I wanna throw my 2 cents in... First, I'm no major tech dude that knows how to develop or do not use my phone for gaming... just like shiny new things and know how to work my way around flashing and modding a bit...
With that being said, I currently have both phones.
I bought the Nexus 4g at launch, even though I have had terrible luck with Sammy. I came from the OG Evo, thinking I'd try it for 30 days and return it if it didn't knock my socks off.
Needless to say I still have it, and gave my wife the EVO... this phone is amazing. The Super AMOLED screen is much better looking than that of the OG EVO in regards to colors, sharpness, and overall appeal. The blacks are REALLY black, and all the other colors are true... not faded/washed out. The speed of the phone running ADW ex is head and shoulders above that of the OG EVO, and the fact that there have been no bugs with GB and the rest of the system apps is amazing. Like someone above already said, its not somebodies modded GB, its true GB.
I got the EVO3d early as I am a Premier member to try it out... was really curious about the SLCD, 3d, and dual core... plus I wanted to see what an official version of Sense 3 was like. I like this phone too, but have not made it my primary phone. The screen is much nicer than that of the OG EVO, and I will say that it seems more crisp than the Nexus... but the colors are still not as vibrant as the Nexus. It is fast, but I don't know how much faster than the Nexus. Scrolling does seem to be smoother, but I think that is a result of the material used on each devices' screen.
I want to like the EVO3d, I want to put it on my primary line to really test it out... but everytime I really think about doing it... I look at my Nexus and decide against it. One because it's naked, as I don't want to buy a case for something I most likely will return, but mostly because I don't want to not use the Nexus as much.
I actually googled Nexus 4g vs. EVO3d to look for other's opinions... I was curious to see if there are many other people thinking the same way I am, or if I just really need to buck up and put the EVO3d as my primary phone in the next 20 days.
I guess we all need to take a step back, forget about the internals of the phone, and really see what is the best Android phone for what you need it for...