Wow, I know these are fund raiser rides, but $90 for a long century seems like a lot.
I do like the Windy Ridge side. Traffic is virtually non-existent. Views, when they do open up, are stunning. The forested sections are pretty neat too. The roads can be quite chunky over there in places and you have to be careful on parts of the descent (some rutted pavement and the rest is rough chip seal). I did the very first HPC and it did not feel competitive but I only really rode with a group for the first 20 miles or so to the start of the climb.
I have not ridden the Johnston ridge side.
I would guess the weather in September has been 50/50 but probably better than May.
Which ride has a more pleasant drive for you? Another solo option is to ride Windy Ridge from the south, starting from Cougar. Or if you have a driver who can drop off and meet up, ride from Cougar to Packwood.