comment: Well first off, the POTUS office isn’t one-to-one with a prime minister of a parliamentary democracy. The POTUS is the head of state in the US, whilst the head of state in, say, the UK, is the king. The prime minister is just the head of the government, and is much more restricted in terms of executive orders and such than their American counterpart. 
  Secondly, you seem to think that parliamentary democracies don’t have separation of power. They do, just as the US. The government is separate from the parliament, just as in the US. 
  Thirdly, I think you place too much important on individuals over parties and policy. I don’t necessarily care who heads the executive branch, I care about the policies they enforce. And that’s much more dependent on which party they represent than who they happen to be.