Which size fits my valve?

Match the inner diameter to your stem: both are 3/16". Then check clearance for the outside diameter and thickness. The A83 needs room for 9/16" across and 3/32" deep; the A80 needs 1/2" and 1/16".

Is the A83 just a thicker A80?

No. The A83 is both wider (9/16" vs 1/2") and thicker (3/32" vs 1/16"). The bore is the same 3/16", so they are not interchangeable where space is tight.

Both are 100% wool felt from the same origins, so wear depends on fit and use, not on the model. A washer that is too thick for its seat compresses and wears faster, so correct sizing matters more than the number.

Do they come in the same colors?

Yes. Both come in white, black, green and red, sold in dozens or hundreds.

Is it worth stepping up to the A83?

Only if your seat takes the larger size. If the 1/2" A80 fits, the wider A83 adds nothing and may not seat.

Wool Felt Valve Washers 9/16" vs Felt Valve Washers

These are the same 100% wool felt washer in two sizes, so the choice comes down to fit, not quality. The A83 is wider at 9/16" outside diameter and thicker at 3/32", while the A80 is 1/2" outside and 1/16" thick. Both share the 3/16" inner diameter, come in the same four colors, and are made in Italy and Germany, so owners of either get the same felt. Buy the size that matches the valve stem and the space it sits in.

Choose the A83 if your stem needs a wider, thicker washer and the seat has room for a 9/16" outside diameter at 3/32" thick.
Choose the A80 if you need a slimmer, narrower washer at 1/2" outside diameter and 1/16" thick.

Prices

Wool Felt Valve Washers 9/16": $8.00 at Ferree's Tools Inc (last seen Nov 10)
Felt Valve Washers: $8.00 at Ferree's Tools Inc (last seen Nov 10)
Difference: both cost the same at the latest prices (Nov 10)