When it comes to deciding between a rocker or a glider, it's really about your preference. So long as a chair moves back and forth, your baby is unlikely to prefer one over the other. You, on the other hand, will be the one seated on the actual chair, so it's important that you're comfortable sitting for long periods of time.
One important difference to consider is that rockers tend to move back and forth at a faster pace and may be less sturdy than gliders. For this reason, many parents opt to go the glider route as opposed to purchasing a rocker for their nursery.