I've only been to Allentown once(First Frost). I mainly chatted up with Gun Runner John, Ed Boyle, and a few other folks as well as my local dealer. It is indeed a good show with a good amount of options depending on what you are looking for. Even if you are looking for something specific, it may turn up there depending on how new or how old.
As Sean stated, a good mix of new and old. The running layouts are pretty great too. I think the S Scale one in 2019 was the better of the layouts at that show if I recall. That might be because they were running that from the start whereas the other two(or three) I think had stopped for a bit(while I was there that is).
I didn't find exactly what I was looking for(being that it was a specific engine), but I did get some excellent deals on rolling stock from two of the dealers.
Also, John gives great advice if you run into him. He was talking to me about soldering(which I am terrible at) as well as one of the brass Williams engines(Niagara I think) that someone was looking at. I think he may have mentioned a Williams J during our conversation, but not entirely sure and I wager John probably slept since then as well.
Regardless of me carrying on, it is a great show. See if you can get in on breakfast. Being that I had a ways to travel and couldn't get out of bed, I missed out on breakfast.