It's actually 8% per year.
I figured I'd rather have the money now, especially since I can invest it at 7%, such that there is absolutely no advantage in waiting.
Yes, but you can get the 8% at essentially no risk at all. You might do better during the 4 years, but there's at least an equal chance you will do worse.
The real advantage is not the payments you get up front. It's the larger payment you will get for the rest of your life.
If you expect to live past 81, waiting until 70 is the better deal. If you don't expect to live past age 81, starting earlier is the better deal.
Of course, there are other factors. For instance, do you have enough assets to maintain your desired standard of living until you are 70? And, will you seriously deplete them, leaving no room for error?
And, how much income do you get from other sources? If Social Security benefits are means-tested (even Christie has advocated this), you might do better or worse, depending on when you receive the income, and how much.
And the real wildcard: what happens with Social Security in 2033, when the Trust Fund is exhausted? If nothing is done, benefits have to be reduced about 23%, by law (as that's all the income taxes will sustain). If you start earlier BEFORE the reduction, you might do better.
There isn't an easy answer. There are two many unknowns, and any answer is not "one size fits all".
/mark