We don't. People that know better have been saying for a while that larger sensors are better (larger pixels as Apple put it). More pixels in the same area tends to be worse.
64-bit means a few things practically, and they all stem from the way addressing works. 64-bit as compared to 32-bit can speak to (or: address) more RAM, and can also interpret longer (or: more complex) instructions. This usually comes with a few trade-offs, it's going to be a more computationaly expensive CPU (or: more processing overhead).
At the moment, there probably isn't much of a benefit. We don't know how much RAM the iPhone 5S has, but I doubt it's beyond the limits of a 32-bit processor. Also, the only applications that will really benefit from 64-bit cpu instructions are likely to be the really complicated graphics applications. I'm not sure, but maybe Open GL ES 3.0 requires or supports 64-bit instructions. I don't really see anyone doing serious engineering on the iPhone/iPad, so it's likely only going to benefit games in the near-term. If it was equaly as fast as it's 32-bit counterpart, it would actually seem slower for regular tasks; however it's speed increase will likely more than compensate for that.