GeologyRocks

Since most metamorphic rocks come from sedimentary rocks originally  the chances are that low grade are more porous than high grade.  They would not be so dense.

I think that the low grade would weather quicker for the same reason.

 However, I feel sure there would be exceptions.

John

“Civilisation exists by geological consent, subject to change without notice.” -  Will Durant

When you say "low-grade" I'm assuming you are referring to low temperature/pressure.  Sedimentary rocks which have only experienced a slight amount of metamorphism will retain most of the characteristics of the original rock, with the corresponding variance in weathering.  generally speaking, the more metamorphosed a rock becomes, the more it recrystallizes and becomes harder.  So for example a shale can metamorphose to slate (relatively low-grade) and thence to schist (high-grade) with corresponding degrees to resistance to weathering.  But there are always going to be exceptions: for example a hard limestone can metamorphose to a dolomite which could in fact be softer than the original rock.  Other factors to take into consideration include folding and faulting due to metamorphism and other stress features which affect the ingress of water and other weathering agents.

Any rock can be metamorphosed, igneous, sedimentary or metamorphic and unfortunately there's no simple answer to the question you raised.

Thanks to you both for your replies. 

I'm speaking of two distinct metamorphic formations: schist (high-grade) and meta-graywacke (low to medium grade). The thing is that I have run porosity and water absortion tests and found that the schist has greater porosity and water absortion capacity than the meta-graywacke. Both have very low values (as expected) but the schist presents roughly double the value than the meta-graywacke.

Also, if you can point me to any references that discuss these issues (porosity and weathering in metamorphic rocks) I would be extremely grateful.