1.- The Zeus has a 3.5-3.6a (some 900mah and most 1300mah are set for 3.6amps) limit so technically the Zeus can do 18w. Not many juices can though, that's the real limit on a device. Personally use either single coils at 2.5ohms or above or dual coils on the Zeus. Prefer the dual coils but keep in mind they will drain the battery faster than the single. As always with vaping comes down to personal preference.
2.- It's easier to fill a smileo than a tank, but it's not that much easier. With tanks you can keep the tank for a very long time, only suggest recycling them when either the chrome finish starts to peel on the caps, you break the tank itself somehow, or if the o-rings have worn out and you can't find replacement rings/don't want to replace them. The cartos in a tank won't last as long as a smileo that hasn't been run dry, especially with the dual coils, and unlike the smileo once the diaper on a dual is clogged you can't clean it at all. Again it's a matter of personal preference. I use tanks, smileos, cool carts, wow cartos, and other items every day. Just depends on my mood/device I'm using.