If you're going for HB's, the designated raptor viewpoint is where I watch from, as it gives the best visibility of the valley and therefore the best chance of seeing one - the air space viewable from the first car park is very restricted.
They are less than a minutes drive from each other, so nothing to stop you doing both.
If I was going, I'd be there from 9.00am onwards. Tried yesterday for 1.5 hours, and didn't get any, but the weather was far from ideal.
At the end of the day you'll need luck, but the more people there the better - usually someone will pick one up.