Best bet the 150mm lens for the 4x5 will be the best. And be patient, you can find a 6x9 holder cheaper than that. If you want to see how the coverage compares draw a 4x5 rectangle on paper, then with a compass draw a circle just clearing the corners. the take a 6x17 sized cutout and place it over the circle. It'll probably fit inside the circle. Most 4x5 enlargers have 6" or larger condensors or diffusion plates so your circle should be no larger than your equipment will cover.
For a quick fix just use a glass plate to hold the neg (if the saunders will accept it) and mask it to fit with black paper. This would be a quick way of checking to see if your system will handle the coverage as well by doing a grey non image print with the masked glass installed. Any falloff will be obvious. You could also try using a multiple image strip of a smaller frame size and see how the densitys look at the corners.