Hi there.

For me, I will choose SG2. Why? I love to 'upgrade' 'downgrade' my phone. I love to tweak this and that. So, yes Sg2 seems to be a good choice for me.

Iphone 4. All my relatives are using it. Honestly, it is one hell of a phone. Iphone 4 will always be great. Hands down.

At time point, I will say iPhone 4 is better than SG2.

So, choose which ever you felt most comfortable with. End of the day, you must be happy with the phone.

Summary:
SG2: Modify phone (like modifying cars)
iPhone 4: The phone is already great by itself. No need any changes