The T-Mobile branded version, as well as the S2 Skyrocket, are Dual-Core 1.5 GHz processors, if you're not going to overclock either phone, the SGS2 wins out on hardware alone.
If you plan to do things to this phone, such as customization, both in software and overclocking, go Android all the way. If you want it to just 'work', go iPhone 4S.
My vote would be iPhone 4. I have one, it's freaking awesome. They're kind of expensive but man they are amazing phones! Calling is easy, sound clarity is great, touchscreen is easy to use, I get reception almost anywhere, it looks cool, and you can get like a bajillion and one apps on it. I literally have zero complaints. Great phone.
I would love to give you a comparison on why exactly an iPhone is underpowered, but I fell out of my chair laughing when I realized Apple had NO processor specs on its sell site, and the first stat on its "Tech Specs" was "Color: White or Black"