I've actually been thinking about this lately and it often appears to me that the STEEM-USD charts are much cleaner as far as trends and trading ranges go. If you look at the chart in this post, you can see how those trading channels line up - the uptrends, the plateaus, the range peaks, etc.
It's good to look at both charts, but when one of them is a little messy, the other can certainly come in handy. I tend to look at the STEEM-BTC charts for the initial read, then use STEEM-USD to help confirm it. I'm not sure if one is more right than the other though. I would lean towards the BTC charts mostly because there aren't really any STEEM-USD pairings on exchanges, so the actual trading is all relative to Bitcoin.
I wonder if steem payouts play a role in this. I suspect a lot of people that care about what they make on their posts see the $x.xx and think USD in their head. Maybe the same is true for up-votes too.