I think for community engagement you should pick the comments! This way these posts can be filled with interesting comments rather than just "comment" and actually bring some value into the community through these posts!
Also, if anyone is interested, I'm also holding my own giveaway! By simply following me and being active, you're in the draw to win free Steem every week! For more information, click here
I think majority will prefer winner slots because they wont have to do anything for an extra chance to win but if the entry for these contests are comments, it only makes sense for you to reward quality comments!
Also, if you don't want me advertising my own giveaway here, feel free to tell me :)
I don't care people can do more than one contest. Just don't spam people go batshit over that.