You can choose shoes based on your race target and training distance.
I have both Rider12 and Aero8.
Rider12 is good for training esp LSD and leisure running.
Aero8 is classified as shoe for both training and competition by Mizuno.
I think they are very similar, except Rider12/13 provides better protection, but a little bit heavier than Aero8 (~300g Vs ~250g). I consider Aero8 is just the light-weight version of Rider12/13.
If you would like to have a pair of shoe for leisure running, I recommend Rider12/13.