The Tower is at one end of the City and Westminster at the other so you'll have some travelling to do whichever hotel you choose.

As for the PPs, both are close to Waterloo and a short walk over Westminster Bridge to the Houses of Parliament.

I prefer CH to WB as it is much smaller. PP Westminster is huge, and can feel very impersonal. Breakfast at CH is slightly better as well and the dining room is much lighter and feels less cramped and crowded. Waterloo tube is a 5 minute walk away and there are a good few places to eat on South Bank, again 5 - 10 minutes away.

Hilton Park Lane is a bit away from most of the main tourist spots, albeit in a nice area. Another large hotel and the extras can be very expensive. If you have Hilton status, the lounge is not bad.

Hilton Tower Bridge is very nice, close to the Tower but a bit distant from other places. London Bridge tube is 3 minutes walk away. It is one of my preferred London Hotels.

If you want central with access to restaurants etc. then why not look at Hilton Trafalgar. Right on Trafalgar Square, nice rooms (typically larger than most London rooms), and a much more boutique hotel feel than any of the others.