Personally I would choose firemaking, but do which ever skill you actually enjoy more

You'll enjoy training for a level 99 skill more if its the skill you want to be doing.
However I would recommend that if you are fletching not to train with willow logs past level 70. You can make yew long bows at level 70 and lose no money at all - possibly even make money which you could then spend on firemaking?
If you ever were planning to train both skills to level 99 one after the other then fletching >> firemaking would be a good idea. This is because with fletching you will probably be able to generate money which can then be spent on the logs for firemaking. You can train both skills one after the other without having to worry about where the money is coming from for the next one! ^_^