IME, they're both two different species of SS strings. I tried both on a Jazz Bass and my opinion is based on how they sounded on this bass:
-ProSteels were the smoother feeling of the two, but their scooped tone made it difficult for me to cut through when I was jamming with my iPod. They would probably sound better on a more midrange-oriented bass.
-The Roto 66's were rougher in feel, but I prefer them to the ProSteels on a Jazz Bass because they're richer in midrange.
In the end, I tossed both because I prefer flats, but if I absolutely had to use SS rounds, I would definitely consider these.