Mike, is there more than one subscription option within the radio buttons? I'm assuming that there are multiple frequencies based on the fact that there are 3 options, so it sounds like you're also going to want to retain their frequency selection. I'd suggest a smart campaign listening for that form fillout, and if the radio button they select relates to "subscription", then do a change data value for the subscriber checkbox on their custom lead page.
I've also had some trouble with display value and stored value on the forms, so I keep both of the values the same. Probably something I should reach out to support about...
Hope this helps!