I've got both - if I were using only these two, the Erect would be rhythm and grail for leads (or emphasized rhythms). I think the grail has a higher midrange emphasis (but I haven't looked at an EQ analyzer so I might have that backwards, but that's the way I hear it), but the sonic quality of these two mods is very close otherwise. I very, very slightly prefer the Erect for its dynamics, but if I liked the midrange signature of the Grail more, I could let go of that bias pretty fast.
I hate to say it for the sake of all those sitting on the fence, but this is definitely, absolutely a case where you almost have to have them side by side to decide which you like better. On the flipside, if you haven't heard the other one, you can't go wrong with whichever you have access to - they both kick ***!