There are two things that bug me about tank atomizers and size isn't one of them.
I try to at least take a stab at keeping vaping as healthy as possible. Any tank I've had - carto, genesis, silica - you can literally watch the color of the juice change before your eyes. It might be irrational but my brain has a problem with that. Wicking is a two way street.
For the same reason I like clean coils & wicks. I haven't found a tank system that makes it quick and convenient to clean them whenever I feel like it.
I'll 2nd that. Some are definitely easier than others but I've yet to find one that I'd call easy or convenient. Then again, I like to play with my toys so that's not a major turn off for me.
Since you mentioned the whole "juice in the tank gets dark" thing... I've always experienced that myself, until recently. I've got a genisis setup that's close to a month old and the color of the liquid looks like it came straight out of the bottle. The only difference between this and previous builds is that this one is using a Nextel XC132 wick.
Like you, I always assumed that the dark color was the result of juice from the coil wicking back down to the tank. I'm now thinking that it has more to do with heat than anything else. I suspect that the heat transfers down the wick and it's the transferred heat that darkens the juice in the tank... More so than juice from the coil being wicked back down.
The XC132 doesn't transfer heat so no color change...
I've never used a carto tank but you say the same thing happens with them too... If so, that seems to add weight to the idea that its the heat causing the color change... Just a thought.