I have both. The 9500i has some very nice GPS capabilities for marking known speed traps or traffic cameras and blocking known false alerts. Other nice features over the 8500 is the auto dimming display and auto volume adjustment that compensates for background noise. The 9500i also tells you the radar band via speech, never a need to look at the unit. The 9500i firmware can be updated via a USB port on the right side of the unit. No way to do that with the 8500.
According to Escort, both units have the exact same detector sensitivity and range across all 3 bands, so your choice is going to boil down to features. One thing in favor of the 8500 for motorcycle use is the larger buttons. With heavy gloves on I have found the 9500i's smaller buttons a bit harder to work than the 8500's.
Both units have an earphone jack that allows easy connection to your com system or some other kind of alert system. Neither one is very easy to hear at speed relying on their internal speakers.