All of the poll based markets are currently based off of averages from realclearpolitics.com (RCP), but unfortunately, RCP has proven to be unreliable, untimely, and uncommunicative regarding the polling data they use. Pollster has been much faster with adding polls, responding to inquiries, and they use more polls than RCP. I see no benefit to using RCP over Pollster.com at this point.
This week a YouGov poll was released on Monday. This pollster is used in several of the averages that markets are based on: Obama Approval, Direction of Country, Congressional Approval, and the favorability markets. It's now Friday and RCP has still not posted the poll. Pollster.com added the poll on Monday. Two of the markets the poll is used in ends on Mondays so it's impossible to know if you can rely on RCP to post the polls and make an informed decision in those markets and any of the other markets. Several people (at least 6) have reached out to RCP about this poll and they haven't responded to anyone.
On top of the YouGov example above, RCP has been slow to add other polls as well. The Reuters/Ipsos poll which comes out every Thursday often doesn't post to RCP until Friday afternoon. RCP just added that poll to the Trump vs Clinton average and the market that uses it closes on Thursdays. Other polls have also taken days to show up. Pollster.com has been updating as soon as they get a poll vs RCP who updates in batches or whenever they get around to it.
RCP never responds
As mentioned above, several people have reached out to RCP about the YouGov polled since Monday and they do not respond when contacted via their support page, twitter, or email. I've emailed them several other times over the past few months and they have never once responded to a question. I've emailed Pollster.com twice with questions and they not only responded but they did so the same day.
Pollster.com uses more polls
One big benefit to using Pollster.com on PI is that they use several polls that aren't included in the RCP averages. This means more volatility in the markets and more money for PI. This also makes the markets more active throughout the day and keeps people on PI longer and thus trading in other markets.
Pollster has several other benefits for traders like an open API, cleaner site, and twitter posts on each new poll.