Your fund choice is going to depend on your appetite for risk. Figure out what asset classes you want to hold, and in what ratio. From there, figure out the cheapest way to achieve that balance across all of your accounts.
Completely made-up scenario:
You decide you want 60/20/20 in US/International/Bonds
Say your 401k has a great US index fund option, but the international fund is crappy, and your wife has good options for all three. Only buy the cheap US index fund in your 401k, buy all three in your wife's account, and then balance out the remainder using the cheap index funds at Vanguard. You may not be able to achieve the exact ratio you want all the time due to fluctuations and contribution limits, but you can get close.