I just ran a series of tests with an Amex Plat with no credit remaining, and a CSP.

The fare across a number of routes changed fairly constantly. Sometimes the Plat was higher, sometimes the CSP was higher. Always within $1-2 of each other. Overall, the CSP tended to have cheaper UberX rides, while the Plat tended to have cheaper UberBlack rides.

The more interesting thing I found was repeatedly changing back and forth between cards for several minutes lowered the fares. I'll have to try this again at various points during the day to see if it was just my time frame. If this is part of the algorithm, it could be interesting.