Hi, I have a different take on the meanings of both sentences. They both mean do you want to fight, neither means less of a fight is expected. I know the words would indicate that the second sentence means the fight would be less of a challenge but it is a colloquial term and doesn't mean less of a fight is to be expected. Either is commonly used and mean the same thing.

I actually think it might be the other way round. 'Take on' is used here in the same sense of 'take on a challenge'. The suggestion is that the speaker is fairly large and tough-looking, and that the other person might be nervous about engaging in a fight with him. If you 'want a piece of someone' means you want to hurt them. It doesn't imply anything about the speaker not needing to fight at full strength. If anything, it's the speaker daring the other person to 'take him on' who seems the more confident about winning the fight without much effort.

I personally think you're correct in that the latter sentence suggests it wouldn't take you much effort to deflect their attack. I do, however, think that both sentences imply your superiority over them.