Well, walnut is a more low end wood, then, you could decided that it nice a deep drum to emphasize it. That said, if you like "craky" snares and high tuning, you can do it easily with a deep snare because the wood itself will bring low end tones even when tunes hih.
So, if your choice is to emphasize the characteristic of the wood and you intend to tune it at F3, F3 SHARP or G, go for the 7". If your goal is to crank it because you like that, tuned at A, A3 sharp and even B3 (Which make sense tof a 5" shell), go with the 5", you will not lose lower end because, instead of being brought by the depth, it will be brought by the tone of the wood itself.
I have the mapex Walnut 30th anniversary, I could apply easily both opposite (Ok, it's a 6,5"), ditto with my walnut 15X8, so, depending on the tone of the wood, you have several strategies. Below is the 3 types of tonewood, low, medium and high:
I went with a 6” depth when I ordered mine . I bought #4 from the very first batch of walnut shells they made . I do not regret my decision to order that size . It is an incredibly versatile depth those 6” snares