As per my previous note "both settings have to be exactly the same" so you need to control it by setting both nodes motion blur settings to exactly the same thing.
Quirky, I know, probably that way because I believe the particle system predates the current full 3D system by a fair long way . . .
And yes it will take a fair bit of by eye tweaking and possibly beefing up in the 2D comp to get it to look right. Bear in mind that basic particles are points with no real size, so motion blur is iffy at best. For motion blur it's often better to use the blob sprite and tweak the size and colour to taste.
There's some good particle stuff here
http://www.steakunderwater.com/VFXPedia ... Simulationmostly by Gregory Chalenko who is a real wizard with the fusion particle system.
And here
http://www.steakunderwater.com/VFXPedia ... le_Systemsa lot of the links are broken and they are mostly on old versions of fusion but some of them still work, as the particle system hasn't changed for yonks.