Depending on how it's wired (presumably an RC filter, if 2 of the 3 lugs are going to ground) the 10nf will focus on or around 650Hz to 3,385Hz and the 15nf will operate on frequencies between 429Hz to 2,257Hz.
Obviously there's a lot of overlap, but of the 2 I'd plump for the 10nf.
You should of course wire the 10nf in parallel with a 2nd, 2nf capacitor to achieve 12nf, your frequencies would then be 536Hz to 2,281Hz.
Another option would be to use a variable capacitor, (a little trimpot) but I'm not sure there's enough room on the board for anything lie that...