My favorite enlarging lens for med format, esp 6x9, is actually a 150mm Apo Rodagon N, something ordinarily prescribed for 4X5 (sometimes I use it for that too). Why? That way I'm using just the center of the optic, making it proficient at even the widest stop if necessary. But more importantly, this gives me perfect illumination of the field, so that standard burning-in of corners and edges is not even needed - one less nuisance. Scale this up, and you can understand why I often use a 180mm lens, over even a 240mm, instead of 150mm for 4x5 film, though I have distinct other reasons for keeping all these various lens options on hand.
Now I will admit that with somewhat smaller 6X7 originals this habit can lead to an annoying amount of distance between the enlarger head and the easel when using a 150mm lens, causing me to defer to a 105 Apo Rodagon N instead, which still in my opinion is preferable for 6x9 to a 90mm lens. But a 90 would in turn be preferable for 6x7 work to a 75 or 80mm, and so forth. Medium format can imply very different things in this respect. 6x9 is twice the size as 645.