Alfy, Quick Magic shouldn't be included in the same thought as Cure Potency and -Enmity. Quick Magic only has an effect at the precast stage, nothing during actual casting.
If you're a devout WHM and value keeping people alive as best you can, Janniston is your ring of choice. The majority of your WHM career will be casting single-target cures, which 99.9% of the time will never fail you at max Fast Cast cap.
With your current gear, the Fast Cast on Weatherspoon will not affect the casting speed of any Cure or Curaga, and the only times you will really notice a difference between normal cast and Quick Magic cast is on Curaga III+.
Say your Cure VI heals (base) for 800. With natural 50% cap, that is 1200. With your earring, 1216. With Janniston as well, 1256. So, 40 HP, doesn't seem like a big deal right? Well, with Afflatus Solace, that 40 is actually 40 HP and 15 Cureskin. Imagine Curaga V, now that 40 could be more, 80, 120, etc. It adds up and can mean the difference between life or death.
Weatherspoon might be more versatile and provide "more" benefits, but if you want to devote to the craft, Janniston is the better option for a career WHM.