There's a lot of speculation about the SoC used in Samsung's Galaxy S II, thankfully through process of elimination and some snooping around we've been able to figure it out.
We know for sure it's not NVIDIA's Tegra 2 or Qualcomm. That leaves Samsung or TI. A quick look at GLBenchmark2's output gives us the GPU string: ARM Mali 400. TI's OMAP 4 uses a PowerVR SGX, so it's out of the running. This leaves one and only SoC: Samsung's own Exynos 4210 (formerly Orion).
Exynos has two ARM Cortex A9 cores running at 1GHz. As a result, general performance of the Galaxy S II is competitive with phones based on NVIDIA's Tegra 2. The Galaxy S II runs Android 2.3.1 compared to 2.2.1 used by the Tegra 2 phones, and as a result has better Javascript performance which we see in some of our benchmarks.
| Physical Comparison | |||||||||
| Apple iPhone 4 | Samsung Galaxy S Fascinate | LG Optimus 2X | Motorola Atrix 4G | Samsung Galaxy S II | |||||
| Height | 115.2 mm (4.5") | 106.17 mm (4.18") | 123.9 mm (4.87") | 117.8mm | 125.3mm | ||||
| Width | 58.6 mm (2.31") | 63.5 mm (2.5") | 63.2 mm (2.48") | 63.5mm | 66.1mm | ||||
| Depth | 9.3 mm ( 0.37") | 9.91 mm (0.39") | 10.9 mm (0.43") | 10.95mm | 8.48mm | ||||
| Weight | 137 g (4.8 oz) | 127 grams (4.5 oz) | 139.0 grams (4.90 oz) | 135.0 grams | 116 grams | ||||
| CPU | Apple A4 @ ~800MHz | 1 GHz Samsung Hummingbird | NVIDIA Tegra 2 Dual-Core Cortex-A9 (AP20H) @ 1 GHz | NVIDIA Tegra 2 Dual-Core Cortex-A9 (AP20H) @ 1 GHz | Samsung Exynos 4210 Dual-Core Cortex A9 @ 1GHz | ||||
| GPU | PowerVR SGX 535 | PowerVR SGX 540 | ULV GeForce @ 100-300 MHz | ULV GeForce @ 100-300 MHz | ARM Mali-400 MP | ||||
| RAM | 512MB LPDDR1 (?) | 512 MB LPDDR1 | 512 MB LPDDR2 @ 600 MHz data rate | 1024 MB LPDDR2 @ 600 MHz data rate | 1GB | ||||
| NAND | 16GB or 32GB integrated | 2 GB, 16 GB microSD (Class 2) | 8 GB integrated (5.51 GB internal SD, 1.12 phone storage), up to 32 microSD | 16 GB integrated, up to 32 microSD | 16 GB integrated, up to 32 microSD | ||||
| Camera | 5MP with LED Flash + Front Facing Camera | 5 MP with auto focus and LED flash | 8 MP with autofocus, LED flash, 1080p24 video recording, 1.3 MP front facing | 5 MP with autofocus, LED flash, 720p video recording, VGA MP front facing | 8 MP with autofocus, LED flash, 1080p video recording, 2MP front facing | ||||
| Screen | 3.5" 640 x 960 LED backlit LCD | 4" Super AMOLED 800 x 480 | 4" IPS LCD 800 x 480 | 4" PenTile LCD 960 x 540 | 4.3" Super AMOLED Plus 800x480 | ||||
The GPU accelerated UI used in Android 2.3.1 makes the Galaxy S II feel a bit faster than the Tegra 2 phones, however that's not always the case. While web page loading feels comparable between the Atrix 4G and the Samsung Galaxy S II, Tegra 2 appears to handle flash a bit better than Samsung's Exynos.
This is a pretty significant difference in our Flash benchmark, however it does translate into a somewhat less smooth experience when scrolling around web pages with Flash.
We managed to run GLBenchmark2 on the Samsung Galaxy S II and compared it to our recently reviewed/previewed Tegra 2 smartphones.
The Mali-400 MP performs pretty well in GLBenchmark2, however it's still a bit behind NVIDIA's Tegra 2. Note that the Galaxy S II runs at 800 x 480 so its direct competitor in this case would be the Optimus 2X. These results don't tell us a lot about the GPU's performance other than the combination of hardware and drivers isn't quite up to par with what NVIDIA has today - at least under GLBenchmark2. There's so much that can be done with driver optimizations that it's difficult to draw any meaningful conclusions yet.
Our impressions with the Galaxy S II are very positive, it's much improved over the Galaxy S. Probably the most immediately appreciable change is the completely different screen and in-hand feel. Samsung's Super AMOLED Plus display does away with the PenTile grid that graced the original set of AMOLED displays, and instead uses an RGB line array just like a normal LCD. 
At the same time, Samsung is advertising response rates of below 1 ms, improved gamut, and viewing angles. From what we saw, Super AMOLED Plus was impressive, and having a 4.3" screen doesn't seem unmanageable compared to 4".
The backside of the Galaxy S II is completely changed - thank goodness. Gone is the super-slick and scratch prone glossy plastic. In its stead is a textured surface that won't show aging nearly as much. The entire back doesn't peel up to reveal the battery, just square shaped area. During our testing and just playing with the Galaxy S II, the phone seemed to get inordinately hot, much hotter than I remember any other smartphone getting. It's also impressively thin - just 8.48 mm compared to the already super thin 9.91 of the old Galaxy S, and thinner than the iPhone 4's 9.3 mm.
Almost all of the button placement is the same as the old Galaxy S, what's curiously different is the presence of a center home button and removal of the Android search button. It's amazing how integral the search button really is - in some contexts (namely kwaak3) it's the only way to bring up menus since it used to be a button every Android device was guaranteed to have. Whether this is something that will change in carrier-specific versions is something that remains to be seen.
The Galaxy S II has an HSPA+ baseband with 21.1 Mbps downstream support. It's unclear whether any of the networks here at MWC have HSPA+, so although we ran speedtests, we're not sure if they're representative.
