Hard to say. They were at this peculiar point where both courses of action would be beneficial for them. Personally, I think they should have sold it. I'm usually the one that is pro-founder's ownership and control. Nevertheless, there aren't many barriers to enter the coupon market and as much as I understood Google's strategic reasons for wanting to buy Groupon, I've always thought it was overvalued for what it offers as a service and because of how easy it would be for a newcomer with a better take on the group coupon phenomenon (think Hipmunk in travel industry).
I think it was a bad move since Groupon isn't a company that can really expand it's offerings. What do you think?