Neither. Mayonnaise or Tartar sauce for pure win and a squeeze of lemon or lime.
The only thing I'll put on a fish finger sarnie is some vinegar, I'm not a fan of re... ketchup and brown sauce just seems wrong.
awesome from the top down:
slice of white bread (buttered)
2 rashers of bacon
a couple of spoons of baked beans
1 potato waffle (the beans should fill the holes)
4 fish fingers
slice of white bread (buttered)
Season to taste.