The i simply stands for "improved." When they upgrade an existing model, they designate it with an i. After that, they generally replace it with a totally different model.
In the case of the descent i vs. the descent, you are mainly talking about a lot more power: three 250 watt amps (2100 watts peak) vs. just one 400 watt amp (800 watts peak) to drive the three 10" woofers. There are other differences too, but that is the main one.