The 8500 with its flared tip is better for what it is intended for, moving piles and piles of leaves. The 7500 with its pointed tip is better for what it is intended for, pin pointing on grass and debris.
In fact, today I was thinking this very thing. I purposely through tons of grass clippings into a parking lot and tested both the 8500 and 7500 side by side. The 8500 of course blew a wider swath. The 7500 blew a slightly narrower swath but deeper. Both are POWERFUL. Also, I ended up driving my mower over the grass clippings, which makes some of them stick to the concrete... The 7500 can "unstick" the smashed grass better than the 8500 because of its higher velocity and pin pointed nozzle.
The 8500 is undeniably moving more air, you can just feel it has more "kick." But, in REAL WORLD applications, they are almost the same. The 8500s strength deviates more towards leaves, and the 7500s strengths deviates more towards debris and grass.
The 8500 is noticeably taller and bulkier that the 7500 also.