I bought a Nexus 5 32GB the day it was announced. I've had it for a little over one week now, and I really like it. Great as the HD2 is, today's software seems to want more CPU and RAM to work well. No matter how much tweaking I do to the HD2, it still seems underpowered. The Nexus 5 has enough horsepower that I think it will last for a while, hopefully as long as the HD2 has.
The first thing I did on getting the new device was to root it. I used ChainFire's root method from XDA and it worked the first time.
NYLimited is right about the SIM. The N5 uses a micro SIM. I got mine for free from a T-Mobile store.
Good luck with the new device.
Yes, as you said, today's software wants more CPU and RAM to work well.
I can't wait to get my Nexus 5 and root it.