Not enough information. Is the fowler a single barrel or a double? Is the AYA a 12 ga. or a 20 ga.? Options, wood, dimensions...? Condition of the custom fowler? Both prices sound very good for guns in great condition.
I'd be leaning toward the custom flintlock fowler with an eye toward how much I could get for the AYA 25 if I bought it as well for potential resale. You could end up with a Ken Netting fowler for much less than $1200 if you could sell the AYA for a profit.
The AYA is a 12 the lower part of the comb has a gouge from being dropped on a shoe rack over all wood I would put at good with light handling marks my guess 90 plus. Mettle and barrels at 95 plus in fact it still looks new
I have the sister gun from the original purchase. What is killing me is I know the cost of what we payed back in the 70s which was less than a $100.
The fowler was made around the same time period single octagon barrel. I know Ken is 3 years behind in work, and a 3/4 scaled piece from him is several thousand more. The condition is 95-99 % o scratches or dings all Kens flint locks look like they are 200 years old. I'll work on getting pictures