I've done both. The route via the Siberian pass junction and Rock Creek trail is _much_ nicer than the PCT. The PCT isn't bad and has some more expansive views to the South and West, but Rock Creek is a really beautiful hike with plenty of water, some very good camping, and another couple of lakes to swim in.
It also gives you the option of stopping 3 miles early at rock creek lake where there is (or at least, was 2 yrs ago!) a bear box and decent camping, very under-used.
I'm not even sure if it's as much as 0.5 miles longer, but even if it is, the scenery will easily distract you from the extra distance :-)
For anyone going up Whitney, I strongly suggest moving beyond the horrendously over-used Guitar Lake. What I always do, ascending from the West, is to cook dinner at a nice spring about 0.5-1 mile after Guitar, and then head up to the spectacular sites just below the junction of the JMT and Whitney trail. Although dry (absent snowmelt), these sites are stunning, and put you well ahead of any crowds the next morning.
