When we had our first (who was the first grandchild on both sides of the family), my husband was insistent that we had to be "fair" - my mum shouldn't come for longer, and shouldn't come earlier. After much discussion, we agreed that both grandmothers would come about 2-3 weeks after my due date and stay for 2 weeks. It worked out well, but I wished I'd had more time with my mum separate from my MIL (with whom I don't have a close relationship, even though it's mostly "civil").
2nd time around, after a difficult and complicated pregnancy, we agreed that my mum would come over a few days before my due date and stay for 2 weeks - that way she should be here for the birth. I'm glad to have had that time with my mum before the baby came and to have her around when the baby was born. My MIL, it was decided, would come a few weeks later when our baby was about a month old. As it worked out, our daughter was still in NICU and honestly, I wish my MIL had not come then as it added more stress to our already stressful situation. I didn't find her supportive and I was in the hospital every afternoon anyway.
I know that my situation is different, particularly with my 2nd daughter - but ultimately I think you need to decide what will work for you and your husband. I don't think choosing the "fair" choice is always the "best" choice, choose what works for you. Who will support you through that time? Who is likely to stress you out more? Yes, their feelings should be considered but I think you need to choose what will work for your family, whether it's lots of people around there and then, or whether it's waiting a couple of months... or even if it is waiting til the baby is born to make any plans... do what is right for you guys.
If I had baby #3, I would probably want my mum around there and then, and my MIL around when the baby was a month old. If it was a complicated pregnancy again, I would want my mum there when the baby is born, and I'd ask my MIL to wait and see and if there is NICU again, I would prefer her to wait until after the baby was out of NICU... not "fair" but that is what is supportive to us as a family (and my husband by now has seen that my mum supports me and his mum stresses me, so now he supports me in those decisions too...)