Progrid Omni 8 is a departure from the 7 version in that the 7 had both moderate and ultimate styles depending on how much stability you wanted. The Progrid Omni 8 is closer to the 7 moderate version in medial EVA firmness as shown below.
The other big difference is sizing. Progrid Omni 8 runs 1/2 size shorter than Progrid Omni 7. Therefore, if you’re used to wearing a size 10 in the Omni 7 then you will want to size up to a 10 1/2 in the Omni 8. Below is a comparison length between Progrid Omni 8 W and Asics GT-2140, both in women’s size 8. The 2140 is true to size whereas the Omni 8 is not.