The Evo is using the same system on a chip as the N1. The Evo has a battery that is a tiny bit bigger(1400mAh vs 1500mAh). It uses a screen that is both larger and uses more energy per square inch than the N1 screen. I don't think it is far fetched to predict the Evo's battery life to be worse. I'm well aware that the N1's battery life is not fantastic as is(I just bought a spare battery for mine the other day), which is why I was pointing out that it very well may be an issue on the Evo.
They both use Snapdragon, but that's really where the similarities end. Different screens (both size and tech), and also very importantly, different radio technology. While its not necessarily far fetched to compared their battery life, these things are hard to judge. My HD2 gets as good (and sometimes better) battery life as my Droid, despite a bigger screen, faster processor, and smaller battery.
My understanding is that AT&T will eventually be releasing the 21mbps version as well, just not as soon as T-Mobile. The reason these two companies are not rolling out LTE 4G soon is because their GSM technology still has a lot of room to expand, significantly more than CDMA does, which is why Verizon and Sprint are rolling out new technology so quickly.
I could be outdated, but last I heard, AT&T is not committing to a anything beyond 7.2, and they expect to move to LTE in 2012. I understand that HSPA+ has much more room for growth than EV-DO, which makes it necessary for Verizon/Sprint to move to 4G faster....but that doesn't change the fact that 4G still has higher potential speeds than HSPA+.
I'd argue that the AMOLED screen's benefits far outweigh the one negative. I've played with the HD2 for a while at my local T-Mobile store and it is an amazing device, but I can't say I would take that screen over my N1's screen hands down. It's a tough call.
I have a ZuneHD so I'm familiar with AMOLED screens. Yes - it looks great indoors, colors are brilliant, black levels are fantastic, but outdoors, its a big, big dropoff. Like I said...AMOLED is the future, I'm pretty excited to see tecnologies like Samsung's SuperAMOLED get more traction, but right now, they both have their ups and downs.
Also, while I mentioned the size difference in regards to web browsing, don't discount it for typing, either. I'm one that generally hates on screen keyboards. The extra size on the HD2/Evo makes a big difference there, too.
Don't get me wrong - the Nexus One is still a very good device. However, in the world of cell phones, you are lucky if you remain king for more than a few months - and the Evo is coming out six months later than the Nexus One. You have to expect it to be superior. And honestly, I consider the Evo to be on the tail end of this group...we're going to see dual-core 1.3Ghz snapdragons with better graphics chips & SuperAMOLEDs by the end of 2010/early 2011 that will blow the Nexus One/Droid/HD2/Evo out of the water.