It's hard to describe the actual difference in feel, because trying to describe the feel of a stick in words is like trying to describe a particular dog over the phone. You know what your dog looks like, and could look at another dog with similar markings and still know it's not him. But try getting somebody on the phone to do the same trick, just with a verbal description- Good Luck.
What I can do, is describe the physical differences between the two sticks. Hopefully, this will give you some idea of the ways the feel will differ, based on your own experiences with different sticks. (If you have no experience, than forget the dog analogy above; it's like describing "Green" to a blind man in that case.)
An original Pac stick has a very short handle, uses a rubber grommet for resistance & as the pivot point, and activates smooth, silent leafswitches.
A Reunion, on the other hand, is basically a 4-way restricted Super with balltop handle. It uses a spring for resistance, and a plastic cone for a pivot point. It activates standard clicky Cherry microswitches.