Decide on your windlass before buying your chain/size. You can get a windlass for either, but my reccomendation is to buy them in synch. If you did not like the price of the chain, you really won't like the price of a 3/8 chain windlass. Of course, you can always tell the wife what I told mine: You are responsible for weighing anchor! She will be much more amenable to anything that will save her back (and your marraige).
I think the 5/16 would be fine. That is what I bought for mine. I have a Catalina 400, about 22000 lbs before the pots and pans. I will warn you though, I paid for the solid gold chain with studded diamonds, but all they sent me was this crappy galvanized chain (Grade 70, Hot Dip Galvanized High Test).
Enjoy pulling out the credit card. But remember this: the real fun won't start until you try to get 44 tons of chain down the dock in a push cart and try not to let it fall in the water or blow out your back!