Mike
I regularly attend both shows, usually on Sunday. Pretty dumb when they schedule on the same weekend. Some dealers do both shows. I guess the best answer I can give is...you never know. Sometimes my best purchase is jerky. Sometimes you hit a home run.
Harrisburg is bigger and I prefer that show but both have a pretty wide variety although overpowered by the tacticool at times. Still some old time dealers with old time guns but seem to be getting fewer every show. Real bargains are rare, but do occur every now and then. Usually some nice guns though.
You could probably hit both in one day if you get to Harrisburg at open. Valley Forge is right off the turnpike so you could hit it on the way back.