If you are using the tinsel to create bodies go with the larger Size 12. For small flies, tinsel tags on salmon flies, and other tasks where you don't need to cover a large amount of area go with the Size 14.
Tying Tip: A single layer of tinsel often looks bumpy and has gaps. For best results, tie a double layer of tinesl. Attach the tinsel near the head of the fly, wrap back to the tail position, and then wrap back up to the head.