There is always a bunch of discussion about how important the 'total viewers' ratings number is with respect to 18-49 viewers. However, it also seems to me that the number of unique households is important since so many products (everything from cars to dishsoap) are likely to be one product per household.

Consider the following ratings example from last week:

[B]
SHOW                            Households   Viewers [/B]
EXTREME MAKEOVER:HM ED-7P ABC   2605         3480 
DESTILANDO AMOR THU UNI         2590         4704 
FAST CARS & SUPERSTARS 3(S) ABC 2514         3122 

These three programs are seen in almost the same number of households. However, the Univision show is seen by more than 35% additional viewers over the ABC shows. This is an extreme example, but in general the Spanish language programs have much higher numbers for viewers per household.

Does anyone have any insight into how much advertisers care about viewers vs. households?

I have wondered about the same thing.

Logic (not always associated with the TV Industry) would tell me that it depends on the individual program and advertiser.

An ad for Disney world would want a families, thus higher viewers to household ratio. But Laundry soap, would be more intersted in household numbers. A Bubble gum company wouldn't care as much about households, just how many viewers saw the ad.