I've used both Speed Passion and Orion and they are both nice and smooth. In my experience the Orion is able to run happily in a wider range of gearing, is silky smooth at the bottom end and revs like the wind above (my Orion 6.5 is quicker, but smoother, than my Speed Passion 5.5). Nothing would convince me to change now.
I would have a look at the new (now legal) orion motors as they are simply awesome, just remember if your after a 7.5 try it might be worth trying an Orion 8.5.
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