Hey Chris...I'm in the same boat as you. I going to begin spraying Captan this year also. I looked into this and I think I remember that the 50WP was the better deal as far as coverage per dollar. I know that I found them close in price but the WP was a bit cheaper I thought. Where are you considering buying from?
Keystone pest solutions has the 50WP for $32.95 and with shipping is around $45 to your door. The 80WDG I think takes more product to the gallon.
Fact check me on this because it's been quite a while since I read up on it. I might have things twisted around.

I'm also interested to know which mixes and sprays better, also which stores better.

Captan 50wp has 50% active ingredient, whereas Captan 80WDG has roughly 80% active ingredient.

If they were both the same price for the same size bags, you'd definitely want to purchase the Captan 80WDG.

It's been a while since I purchased small bags of Captan 80, but last time I did, they came in 6.25 lb. bags, not 5 lb. bags.

If you are using a backpack sprayer to spray your trees, one small bag of Captan 80 will last a long time. Depending on the exact tree size, one small bag of Captan 80 would treat around 100 large trees.

I don't think you will notice any difference in clogging between a WP formulation and a WDG formulation. Neither one really dissolves in water (like some liquid formulations). Instead, the particles are suspended, which is why it's best to have agitation to keep the particles from settling out in the tank. That said, Captan doesn't settle out too fast, so if you get it mixed up good, it should stay mixed up enough just sloshing around inside the tank, as you walk around.

A WDG takes a bit longer to mix up, but the granules break apart in the water to form the small suspension particles similar in size to a WP. It's generally recognized WDG is a little safer formulation than a WP because it's not so dusty, so there less inhalation of the compound during mixing.

Your sprayer has to also be made for powder solutions.

Olpea, thank you for the detailed response, it was very helpful. Will probably go with the 50WP. Appleseed, I found several online retailers offering 50WP in 5lb bags for around $45.00 shipped, that seems be the going rate.