While I don't think that your question is important when choosing your camera, I think it cannot be answered.
Not the Answer
- Both cameras take great pictures with their respective processors.
- Other factors could be more important to you.
- Get the D5100, if you liked to use either:
- Bracketing (for HDR or something else)
- Timelapse functionality (built into the D5100; you could buy an external adaptor for the D3200)
- Swivel screen for video, use on a tripod, or near the ground. (I find that very useful!)
- Get the D3200 if you like more megapixel or a bit of additional zoom (through cropping). Get the D3200 if you want wifi.
Except for Image Quality, I don't think it can be answered (at least by me, now) because we know too little about the two processors.
The Expeed 3 is said to be quicker (but how much?) and "better". However, the D3200 has more megapixels and needs more processing power, so the gain in speed is required! Given that both cameras can shoot 4 fps in burst mode, I would assume that there is no real speed increase that isn't also offset by the higher pixel count on the newer D3200.
Also notice, that one only notices speed improvements if they are above 30%. This is a rule of thumb that is used with computers. If your change in the system gives you only 10% improvement but has some downsides, it is not worth doing.
The image quality is exactly the same. Okay, not true. But unless you know exactly what you want to do (i.e. only shoot portraits), it might as well be the same.
Because of the higher density sensor of the D3200 the individual pixels are smaller and thus you loose high ISO performance and dynamic range. The better processor seems to keep up with it, though!
The following data are from DxOMark and show three categories of image quality of the two cameras. Essentially the values are almost the same. If you decide that good high ISO performance is important to you, that the improvement of the D5100 become relevant to your decision making, I suggest you pick a completely different camera (used D700 for example but see DxO mark for that).
D3200 | D5100
Color Depth: 24.1 bit | 23.5 bit
Dynamic Range: 13.2 EV | 13.6 EV
Low light ISO: 1131 | 1183
Source: D3200, D5100
I have not found any information about the difference in autofocus. I would guess it is about the same.