Personally I think Google+ Communities are useless in terms of promoting your content compared to Reddit.
No. Using Google+ Communities are generally about as useful as spam posting your stuff in Facebook groups and sub-reddits. Even in forums that allow you to promote your content is generally less effective than other methods, such as engaging with potential viewers in the forums of whatever content vertical your channel is about, engaging in the sub reddit about your content vertical, etc.Conquer all fronts I say.
Reddit's definitely a monster though. If anyone only has time to promote on one or the other, Reddit is definitely the way to go. Engagement is stupid high sometimes.