Kenny is right. The show in Monroeville is a lot of fun. It is great to hang out at the Pittsburgh Independent Hi Railers, watching cool trains I can't afford and talking with the guys. Layouts of smaller scales are also there. I like watching the S Scalers, they have an S scale switching layout and a traditional American Flyer layout where they help the kids operate all the accessories. The LEGO layout gets more impressive every visit.
There are a lot of vendors, mostly O Gauge, but all scales are represented. Some good deals are to be had.
I can't compare to Allentown, since I have never been there.