Tegra 3 is Cortex A9, so they're the same. As for your Sensation vs. Galaxy S2 comparison, as Earlymon pointed out, they are largely comparable. Snapdragon S3 uses the Scorpion MPCore, which puts out 2.1 DMIPS per clock cycle. So, a 1.5ghz Snapdragon S3 puts out 3,150 DMIPS per core, 6,300 total. Galaxy S2 is a dual-core Cortex A9 @ 1.2ghz. A9 puts out 2.5 DMIPS per clock cycle, so that's 3,000 DMIPS per core, or 6,000 total. Snapdragon S3 is 5% faster.
Snapdragon S4 will utilize the new Krait MPCore which puts out 3.3 DMIPS per clock cycle. The first confirmed chipset from Qualcomm will be a dual-core @ 1.5ghz, which means 4,950 DMIPS per core or 9,900 total. Tegra 3, which is still A9-based, puts out 3,250 DMIPS per core or 13,000 total. However, you will never see a real-world application use 100% of each core. There are diminishing returns associated with multiple cores. Also, Krait can be clocked up to 2.5ghz in come configurations, so that first model I quoted was their low-end.
Tegra 3 is a relative beast and a harbinger of what is to come, but it will not be faster than Snapdragon S4, OMAP5, Exynos 5, or A6. The above are all using Krait (Qualcomm) or Cortex A15 (the others).
In terms of power usage, the Tegra 3 uses a low-powered companion core for standby and low=power operations. It's on the same 40nm process as Tegra 2, so the companion core and load-sharing over the cores will give it a slight batter edge over Tegra 2. The other chipsets are either 32 or 28nm, giving them a huge leg up in battery life.
Core2Duo would smoke a Tegra 3. The one Nvidia used in their benchmark was an original Core2 from early 2006, not the modern ones we have today. Even then, that benchmark was rigged to favor Tegra 3 and it came to a near tie.
Just a minor correction, but Qualcomm has stated that their first Kraid-based chipsets will in sin single-, dual-, and quad-core configurations clocked UP TO 2.5ghz and all under the name Snadragon S4. The first confirmed chipset will be dual-core @ 1.5ghz, but that was announced in mid-2011. I think they're going to bump that if Samsung truly gets Exynos 5250 out early.
EDIT: I skipped over Shocky's post, no need to quote it when everything is pretty much dead on