They're very similar, however OW is generally better than ATK across a range of criteria. All that being said, if you're picking between the two, go for the one where you genuinely feel like you connect better with the people, are most similar to the culture, and feel you will be mentored / grown the best!
Overall Ranking: Oliver Wyman (#5 vs ATK at #9)
Total Revenue: Oliver Wyman ($2B vs ATK at $1.4B)
Global Prestige: Equal Ranking
Employee Satisfaction: Oliver Wyman (#5 vs ATK at #12
Europe Prestige: Oliver Wyman (#4 vs ATK at #6)
Asia Prestige: Oliver Wyman (#5 vs ATK at #6)