Test both in huge samples on your walls. All color varies with exposure and its setting. Nobody here has YOUR room or your house. Paint the whole wall. There is no better way. In fact, to truly know is to paint all four walls.

Simply white is warmer, but can take on a yellow green cast depending time of day, and exposure.

If you are trying to paint a dim room into the light? I never find that successful. Try a "color" versus white and yes white is a color. white is at it's best in lots of natural light, multiple exposures. . A primarily northern exposure generally is not that.

A "swatch" is several feet by several feet, and on several walls. : ) Best way is LARGE poster boards and move them around.......or just roll. All paint needs two coats, anyway. Your trim choice also matters.

I find Snowfall White very white and I wouldn't use it in a north-facing room.

To warm up a north facing room, use a white in the yellow/orange/red family. Any cool white is going to look very stark in the room. I used Linen White in a dining room with a north/northeast exposure and it works very well. Try that or Ivory White (925) or Mayonnaise (OC-85). Or use an actual pale yellow, like Weston Flax (HC-5) or Windham Cream (HC-6). But the operative word is "try," you need to try out the color in the room on a large sample during different times of day and night to see how it works in your space.

If brightening the room is the aim then before choosing paint make sure you have enough lighting, thta is how you get a brighter space not with paint. As for which white that you need to decide and best to get big poster boards and paint each one with 2 coats of the color you are testing then move those boards around the space at differnt times of the day and see which one works for you. All the lighting should be LED bulbs in 4000K if you want a feel of sunlit room. Wall colr is decided with all of your furniture in mind too.

Snowfall White is a pretty, soft white that is not as yellow as Simply White. It lightened my dark downstairs with vaulted ceilings. The only room in Snowfall White that had a soft yellow-white color is my northeast facing office that does not have vaulted ceilings. We have bright white ceiling trim. We did our adjoining dining room above the wainscoting in a beautiful, light blue (Ben Moore Ocean Air), and that color blended well with our Snowfall White foyer walls. Our oak floors are stained light with weathered oak stain. Without priming the walls, expect to have to put on three coats to fully cover. I tested 6 whites all over the downstairs. Snowfall White worked day and night and did not change colors in different lighting like some other whites.