You vent into your attic? No mould issues?
Yes I vent into my attic.
I have zero mould issues in 11 years.
I have soffit vents all along my roof line and venting plates between felting inside which opens the felting to create air movement.
As for your fan filter question.
the 6” kit comes with an 800m3/h fan regardless of which filter you chose.
the filters are rated at 150x300=600m3/h and 150x600= 900m3/h
If the filter is rated less than the fan the filter will not clean the air effectively so the the only viable choice there would be the 150x600 as the fan is 800m3/h.
I love my rhino pro 150x600, I hardly ever run my system at 100% and my filter has lasted an exceedingly long time with my 460m3/h fan.