GNS & St. Ambrose both have strong, amazing principals. Primary grades at Ambrose have assistants but GNS does not. Class sizes are similar at both. However, Ambrose has 1 class at each grade level whereas Spellman has 3-4 classes per grade level. Up to you which you think is better.
Behaviorally, St. Ambrose is much stricter. In general this is a good thing, unless your little person is fairly active and may have a hard time sitting for long periods of time-GNS may be better then.
Academics are similar. More struggling students at GNS, but also more resources to address struggling students.
Parent participation is required at Ambrose. If you don't make your 30 volunteer hours there you pay $15/hour. Participation is encouraged at GNS, but besides a core group of parents, kind of lacking.