Hi Derek,
Welcome to the forum,
and apologies for the delay in answering your post.
The Pi Co-op doesn't offer a huge number of advantages over the Arduino other than it's more 'integrated' with the Pi. You don't need the USB cable, it saves some space and is generally cheaper than a full official Arduino. I can't say that it's cheaper than a lot of the fake Arduinos (it's hard to compete with Chinese manufacturers on price
)
As for your second question, having both the Pi and Arduino running at the same time with their own programs, yes that is definitely possible, and that is actually one of the key advantages of using an Arduino in your projects. As well has having the Arduino read in data, it can be running a small program to process the data, and perhaps in some ways to respond to it quicker than the Pi could. For example, if you were using the ADC on the Arduino to monitor a temperature sensor, and wanted to take some action (such as turn off a motor) when it got too high, then it would be much better to do this on the Arduino than to pass it up to the Pi which might be busy doing something else.
Now, to actually get the data from the Arduino to the Pi, and to process it there, I would recommend using the Arduino serial library, and then something like Python with
PySerial on the Pi.
Hope that helps. Please let me know if you need any more info.
Regards
Alan