Which one is fastest.
The answer to that question is fming, but that is only if you want to lose a crap load of money. For getting fming fast you have to spend between 20-60mil. That is if you want it fast. If you want it slower then fletch you could spend 2.3m on willows to 99 which os 148k willows (takes forever and you get bored way to fast.) Even if you auto it it will take a couple months straight of autoing. Or 3.5mil on 95k maples. Which again will take you a long time. I would choose fletching because you make money and it isn't as bad.
How long do they take aprox.
Fming takes a couple months of doing it every day many hours a day. Fletching is not as bad but it can be longer if you are choosing the fast way to fm.
How much does Firemaking cost
Fming costs this much...
Willows 146k: 2.3mil~
Maples 95k: 3.5mil~
Yews 64k: 22mil~
Magics 42.5k: 55mil~
How much do you make fletching.
Depends what method you use and all. Usually using yew longbows you make 50k per 1k longbows you do. you have to do 82k yew longbows strung from 70-99. Which would make you approximately ~4.1mil.
Which one has better cape.
Firemaking cape by far is much much better. Everyone has a fletching cape and I think they look stupid. But on the flip side you make money doing fletching and lose a lot by fming. It's all up to you.
Conclusion:
If you want to lose a little bit of money spend A LONG TIME and get a sweet cape pick fming the cheap way.
If you want to lose A LOT of money, take a lot less time then fletching and the cheap way and get a cool cape choose the expensive but fast fming way.
If you want a stupid cape, to take a long time (not as long as the cheap fming) and make money (that's a bonus =) ++++) then chose fletching.
It is all up to you though.