Actually, I think
Wallis is more logical phonetically, but
Wallace should be well enough known that it wouldn't be confusing either. This is going a bit off topic now, but I have a student that I read with every day, and he has trouble with the 'older' names like
Michael and
Richard, not names like
Alexis and
Lynn, which he's familiar with or are similar to popular ones.
Like la-petite-rachel said, it's too closely related to
Wallis Simpson, the Duchess of
Windsor. For that reason, I prefer
Wallis for a girl, and
Wallace for a boy.