Before joining either through an invite from an airline, please read up on e-rewards in this recent FT thread.
Also, on freefrequentflyermiles.com, click on "Other Programs I Like" and then click on "Surveys."
In short, if you join e-rewards via an Delta's invitation, you likely will find the list of available awards restricted. So choose carefully which invitation you accept.

Until recently, at the e-rewards web sit eyou could request an invitation from a sponsor and some of them, e.g. Borders, would not restrict the rewards available to you. At present, however, the site says they are not accepting requests for invitations. I like having multiple options available, but I'm sure for some people, one being limited to one airline is no problem. Plus there's all the other stuff on e-rewards. Recently, 10% off coupons from Sony were quite useful to me, for example.

The thing about e-miles is that once you select an airline from which to earn miles you cannot change it unless you get an invitation from another sponsor and cancel your existing membership, sacrificing any miles not yet deposited in your account, according to the FAQ.

In both cases, the rate at which you earn from doing surveys varies widely from person to person and from time to time. E-rewards, for example, goes weeks or months without offering me any earning opportunities, then sends me such a flurry that I earn $100 of currency in a matter of weeks.

I'd join both, or would if e-miles offered miles that were useful to me.