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.