We are moving to the Lehigh Valley at the end of this month, so over the summer I had the very same question in terms of Parkland vs. East Penn. In the end, we bought a house in the East Penn school district because the actual schools are closer.
Both districts are huge, but the Parkland SD is much more spread out since it encompasses more rural towns, and that means my kids would be riding the bus for a lot longer each day - and with after school activities, etc., I didn't like that idea.
Also, I went to actually see the elementary school my boys would be attending and it was in an industrial park area with tractor trailors whizzing by all day - no good.
I hear a lot about the East Penn HS (Emmaus HS) being overcrowded, but with my boys only 6,4 & 2 I am hoping that that issue will have sorted itself out by the time they get there (or we will have moved again

) Also, they are building another new elementary school (Willow), so that will help in terms of the area's rapid growth.
Pulte (the builder we used) has developments going up in both school districts, and the same model was actually less money in the Parkland SD, but we still went with East Penn because of the schools being closer to where we would live.
I'd be happy to answer any other questions you might have since I just went through the same process. Best of luck!