DOI stands for Double Opt-In, which means that the user needs to verify his e-mail after he signs up.

SOI stands for Single Opt-In, which means that the users only needs to enter his e-mail and that's it - no verification required.

Obviously, assuming two offers are identical, the SOI version will always convert better.

Everyone is going to have their opinion which is better and why.With double opt-in you will get fewer subscribers because a certain percentage will fail to confirm for whatever reason. However, there are those that will say that those that confirm are higher quality / more interested and since they have to confirm can't enter a fake email address.

I personally use single opt-in.

I tell users on the squeeze page that whatever the bribe that they are getting for subscribing will be emailed to them and do not provide a link on the thank you page. That way they know to provide a valid email address if they want to receive it. By doing this I have rarely ever seen someone submit a fake email.

I watch my stats and prune the users that never open any messages or have failed to open a minimum of one message during x time period. By doing this I'm weeding out the lower quality users.

This way I get all subscribers and have a quality list.