If we look at the actual price improvement that retail order flow gets, I think we can be pretty confident that Robinhood and IBKR Pro users, who have the least price improvement today (one group because of greedy brokers and the other because they are usually professionals doing their personal trading), will do better. Users of Schwab and TD Ameritrade, who get the best price improvement, will probably do worse.
Whether the total amount of price improvement in the market increases still does seem to be up for debate - I personally doubt that it will be much better for average retail investors than the old system.
It really depends on whether non wholesale players step up and participate in these auctions.
While Citadel and Virtu are very large market makers, there are other equally large firms that don't jump through all the hoops to participate in wholesale, but would probably do on-exchange auctions.
The SEC also talks about non market makers using this for execution, but I don't think that's going to happen immediately...
Whether the total amount of price improvement in the market increases still does seem to be up for debate - I personally doubt that it will be much better for average retail investors than the old system.