the MP is a great midrange eq and the best guitar eq to my ear. it's not surgical, and the low end is weak as in all manley gear. I dont use the MP on the 2 bus, drum buss or for mastering hardly at all now that I have the TMEQ/Weiss combo.
the GR is a full range eq, with thicker lows and is harmonically more solid/cleaner (no tubes)
If you have no eq and you want something for everything including drums, the GR. If you want great mids and guitar eq, the MP ... if you want both you'd be happy