Portman-Warner is not just favoring PoW over PoS, it's favoring PoW over ALL alternatives. It completely shuts down all potential for new solutions to gain adoption in the United States.
Even if you don't like PoS, you still shouldn't like the Portman-Warner amendment.
Agreed. There are many subtle and complex trade offs that come between PoW, PoS, and alternative consensus mechanisms. But one thing I can say with confidence is that Rob Portman is in no way qualified to understand, let alone make those decisions.
Then make the very first block PoW, which switches to PoS from there after. I'm sure the bill hasn't been written to take into account any duration and so is merely a binary question.
Even if you don't like PoS, you still shouldn't like the Portman-Warner amendment.