If the only months you could go are between mid Sept and Dec. when is the best time to go.

I had my mind made up for mid-Nov but then one of the companies told me that's the worst time to go.

Any opinions on those months?

Where are you looking at going? That may help

OOPS! Forgot to say I'm looking at Tanzania and Rwanda.

Sept/Oct ... Masai Mara will be good with the migration. Don't know about Rwanda i have never been

Northern Serengeti and Masai Mara, you are bound to catch the migration that way.

Haven't been to Tanzania so can't comment on that, but we did Rwanda 1 - 13 November 2007. Perfect time. Take warm stuff - layers of tshirts and long sleeved shirts, sweatshirt/fleece (sandals and shorts never made it out of the case) for Nyungwe, Lake Kivu and gorillas (we didn't do Akagera) - although Kigali was hot. And take waterproofs ... it is supposed to be the "small" rainy season (although we didn't have that much rain).
Enjoy your trip,
Regards Caroline