In this video, Eckart from ADRENO Scuba Diving Melbourne talks through the differences between a rear inflation scuba diving BCD and a jacket style inflation BCD.
Rear inflation BCDS are explained in their name: all of the inflation is positioned on your back. What that's going to help you with is your trim: since all of your floatation is on your back which will put you nice and flat when you're diving.
On the other hand, a jacket style BCD inflates all the around your hips too. This will result in you sitting a little more upright while diving. You compensate for this by using trim weights so you can make sure that you get nice and flat and horizontal.
On the water surface, a jacket style BCD is more comfortable, as you're sitting a little bit more upright. In reality, though, you're buying a BCD to be underwater! Keep the above in mind when purchasing your fist inflation BCD or upgrading to something new.