Nexus has a high definition display and is .35" bigger. Much higher pixel density and the screen is only dwarfed by a couple of HTC phones which don't have the AMOLED tech. Expandable memory is also a non-issue unless for some daft reason you use a phone over a bespoke MP3 player with a better sound card for your music and videos (unless of course we're talking iPhone).
I own a Nexus as well and I have absolutely no idea why I was thinking of picking the S II over it.
Well because the SII has expandable memory, and if you're someone like me who doesn't want to carry around more than 1 thing in their pocket, you get that phone and a phone wallet and job done, I'm saved having to carry my phone + wallet + mp3 player.
Nexus is good, especially designed for 2.4 or is it 4 I forget, but I'd take the SII still... not that I'd be truely disappointed with the Nexus now I really think about it, I just don't see it has having any + points over the SII.
Speaking of which, someone here said that the Android is only for people who like to tinker with their phones, which I don't fully agree with, yes, for any tech savy person, it'd be a bit daft to buy an iPhone, because you'd need to jailbreak it ect. But the Android you can tinker with it without routing around too much, or as much as you want with custom boot roms, YET at the same time offer you a complete experience if you want to use it as just a phone - so why limit yourself with an iPhone.
BTW I'm not a fanboy, I own a hand me down iPhone 3GS completely jailbroken, and it's a nightmare of a thing, it resets itself randomly, its laggy and slow, virtually impossible to back it up properly, but I can run the nexus background (those coloured pixel snakes going over a black background), and can turn off my 3G, 3G data, BT, Wifi, GPS and adjust brightness without having to go through 5 different menus, I can also use Swype instead of the default keyboard, amongst a whole heap of things the default iPhone 3GS couldn't do.
So if you're buying new, get the Android, I would personally wait for the SIII (as I am myself, this is just a stopgap between a E75 and the SIII instead of getting an SII now on a contract and having it be obselete within 3-6 months since its been out for a while now).