I personally prefer the regular plastic carts, and feel they're easier to use, but both work well. The benefit to the capsule/key system is that you can order several sets of capsules, and pre-fill them so they're ready when you need to refill, just pop in a new capsule. You can also do this with the regular refill carts by getting more than one set, to always have a filled set on hand. No matter which style you get, pre-filled carts or capsules should be stored with vent holes sealed, and out of direct sunlight to prevent evaporation and to keep the carts in good working condition. Shake carts and open air vent holes before installing into your printer, then refill empty carts or capsules later when you have time. Both last about a year when used correctly.
I hope this helps.