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I think it's a good idea to be more specific, and certainly adding "sponsored" is very important to be able to do, so that google knows a link is sponsored. Now it can be "sponsored" without "nofollow" and still be fine with google. 
For all my forum links and comment links I decided this was best because it is more specific: "external ugc nofollow."
Google indicates on that page that forums, for example, can use "ugc" to indicate that it is user generated, which is different than just nofollow, which doesn't tell you anything (it could be nofollow because it is sponsored, so now google cannot tell whether it is sponsored or flagged nofollow because it is UGC).
Interestingly google also says that for "trusted" users, for example old forum members who regularly post good links it is ok to remove the "nofollow" on them. So there really should be a setting where you can set the default for all users, but then go in and change them for selected users, or perhaps even make it tied to advancing to a certain group, for example those who have over 100 posts will have the "nofollow" removed.
Just some ideas, but what is clear is that google cannot now tell whether a link is nofollow because it is sponsored, or just UGC, and it is important to tell google this. By adding the UGC tag I've done that.