I will say iPhone 4s.
I do prefer the iPhone 4 S instead of the iPad 2 for the following reasons:
Better HD camera.
Beautiful Retina Display, once you get used to it, you never want to see something else.
It is way more portable than the iPad.
There will be an iPad 3 sometime early next year.
I travel a lot, and lugging the iPad is more cumbersome.
I am spoiled with the Retina Display resolution.
Doing video chat with the iPad 2 is not so portable as it is with the iPhone.
The camera for pictures is insanely great, and has a flash, the iPad 2 has a mediocre camera.
The jump for 3Gs to 4s is huge.
In January 2012, you will looking back at the great pics and videos you were able to capture with the iPhone 4s, and you will regret not having one before.