I think it's better to upvote posts of people that are commenting you. I always try to upvote posts of people that are commenting me because they give me the feedback that I want. I think when you upvote comments you don't help the commenter a lot, but if you upvote his post he wil feel that better and he will be sure that his activity made you upvote him and comment his posts.
Of course if you upvote both comments and posts of people that comments you, it will be much better for them. But to keep your voting power high and upvote as much people as you can it's enoug to upvote their post with your almost full voting power.
I always try to keep my voting power high to give good upvotes to people. Generally when I hit 50 % I wait till it will go back to 100 % and start upvoting again.