... didn't see the need for both the Nexus and the SII... so they chose the Nexus. And given the plethora of high-end smartphones at Version now (Galaxy Nexus, Droid RAZR, Droid Bionic, soon Droid 4, iPhone 4S, HTC Rezound, etc) I'm surprised they just didn't go for it and offer a version of the Galaxy SII as well. It can't be that big a deal to stock a few more phone models. Then again, my local Verizon store was down to stocking only three models of "dumb" phone.. every other model on display was a smartphone. And this a full, large store, not a kiosk.
SIIs have either the Samsung Exynos SOC at 1.2GHz or, like T-Mobile, a Qualcomm SOC. This is based on the 1.5GHz "Scorpion" core, not the ARM Cortex A9 used in most of today's designs. The Scorpion is very similar to the Cortex A8, with some improvements, and of course, the faster clock speed (all A8 designs faster than about 600MHz are at least partially customized, but Qualcomm's pretty much building independent designs, under license). The A9 is about 25% faster than the A8. Scorpion is a little faster than an A8 on integer code, and way faster on floating point, thanks to a fully pipelined FPU. However, it only does limited out-of-order execution, versus the A9's full out-of-order implementation, so it's not as fast per core on integer code. On the other hand, it implements a 128-bit-wide FPU versus the A9's fully pipelined 64-bit FPU implementation, so it may edge out the A9 on some floating point operations (NEON instructions, which can use the 128-bit bus... unfortunately, Android doesn't yet enable NEON). In short, they're probably clocking to 1.5GHz to match the 1.2GHz performance of the A9-based SOCs (OMAP, Exynos) on integer code. Some of the SIIs, the Skyrocket for example, use the Qualcomm chip simply because Samsung's Exynos doesn't have 4G support yet... not sure about HSPA+.
The Adreno 220 or the MALI400 are in the SII designes, GPU-wise. Each is a more modern design than the PowerVR GX540 in the Nexus... but they're also running at much lower clock rates. Clock for clock, either would soundly beat the GX540, but at 384MHz vs. 200MHz or less, it's a pretty close race; I think the MALI400 probably comes out ahead. Unless you factor in the iPhone, which is the only smartphone to date with a dual-core GPU, even though any of these others will toast it on CPU performance. On full screen stuff, I suspect all the Galaxy II win, simply because the Nexus has 2.4x as many pixels.
So many phones are using the OMAP4 because it's the first ARM SOC to support dual memory buses... like most PCs (well, the Intel i7 desktops have a triple memory bus). It's also driving low power DDR3 memory, versus the slower DDR2 still used on many design. The OMAP4460 in the Nexus runs up to 1.5GHz; like Apple on the iPhone 4/4S, Google's downclocking it a bit to improve battery life. The SII will have the same basic power issues; both SOCs are implemented in 45nm CMOS.
Most benchmarks to date are run on browsers, and the Nexus is kicking butt on these compared to the other Android phones. But that's more a browser tweak than pure CPU power (you want both)... running Firefox or Opera, you'll also beat the stock Gingerbread browser. Good news is that a number of the new devices outperform the iPhone 4/4S out of the box... that suggests Google is getting their brower performance together, if not best on a phone yet.
The Nexus camera... so-so. It's better in low-light than I'd expect, and blows away the camera in my old O.G. Droid. But it's still a cellphone camera. So is the highly touted iPhone 4S camera... looking at these in small renderings on-screen, they look great. At full-screen on my 24" monitor, there's no mistaking an iPhone phone for one taken with a real camera. I usually have a real pocket camera with me anyway... these days its an Olympus PEN E-PM1. However, PC World's recent evaluation puts the SII camera as the best in any current cellphone... even beating the iPhone 4S.
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