Dean I would suggest #12 - I use #14's for specific historic items since they are what was used and when making museum quality pieces and that's what the customer asks for that's what I use. But I have also had the fairly frequent problem of the stem bending inside the leather when using #14's no matter how careful I am - with the #12s all such problems ended and they are only about .030" larger than the #14s in diameter so they to the eye they don't look out of place, plus they are usually much easier to find from suppliers.
FWIW - I only use rivets for certain historical pieces such as gunbelts, holsters, and copies of original sheaths that used them - generally sewing was and is better and now as then a mark of higher end craftsmanship..