The difference is in the accuracy of the oxygen sensor. Stock (narrowband) o2 sensors aren't very accurate or precise , so when it comes to making measurements of what your air/fuel ratio is, an inaccurate sensor is going to give you an inaccurate measurement obviously. One of the biggest problems is that they have a small range (band) of air/fuel values for which they are accurate, and then they really suck outside of that range (so they're called 'narrowband'). Wideband o2 sensors have much greater accuracy and precision to begin with, and they have that accuracy and precision over a much broader range of air/fuel values. This is why they are much more useful for tuning and datalogging. It's useless if you're making engine control adjustments based on inaccurate measurements that aren't really precise (repeatable) and it's almost as bad if you're trying to tune off of the datalogging info with crappy o2 sensor readings. You have a performance car, you should have performance sensors.\

By the way, have you ever looked at the BikiRom USB boards from Australia? They're like the Hondata of Nissan stuff and the hardware is really nice.. 1/10 of the cost of the AEM too, although no direct wideband support (still uses the stock ECU narrowband input as far as I know)..