So currently I have a 400D with a 18-55mm kit lens (no IS and IQ is so bad I never use it) and a 50mm f1.8 (I use this all the time). I don't want to upgrade my camera as I plan on going full frame when I have saved up enough money. I'm looking at the 24-105mm f4L IS and the 70-200mm f4L IS. I plan on eventually having both, but out of the two which would you buy first and for what reason? As always, any help is greatly appreciated.