Groupon and Living Social and Google and Facebook are all trying to convert Jane Doe to their subscriber. I find it extremely questionable that Jane Doe is going to be much more likely to come back to Sorbus Spa whether she bought her coupon at one service or the other. Ultimately the client is responsible for delivering goods and services that merit repeat business.
Who does the client go with? Much like with any other advertising the one that delivers the best value. Value here is defined as # of new customers divided by the lost revenue. This will ultimately be driven by scale -- if Groupon can sell more tickets they can offer a proportionally lower margin and will be more attractive then competing services.
I was suggesting the possibility of different coupon services marketing themselves to different segments of the populace; the most common characterization (as far as I can tell) of people who use Groupon is as cheap or stingy, so a service which was able to promote a view of its subscribers as more affluent or using the coupons as a low-risk way of trying something (as opposed to just shopping for deals) would be better placed to market to businesses.
However, you're completely right that clients would go with whichever service offers the best value and best range of coupons, and that scale offers an advantage there. I would add that it is highly unlikely that subscribers would only subscribe to a single service. With the marginal cost of subscribing being so low (just signing up with an email and then looking at the deals when they show up, as I understand it), the increased variety of coupons would almost certainly sway the majority of subscribers. At that point, it would become a fight over which service can find businesses offering the cheapest or most desirable deal, in which scale gives the advantage.
Who does the client go with? Much like with any other advertising the one that delivers the best value. Value here is defined as # of new customers divided by the lost revenue. This will ultimately be driven by scale -- if Groupon can sell more tickets they can offer a proportionally lower margin and will be more attractive then competing services.