The 2nd gen Nexus 7 got the same weight as the Tab S 8.4 or to be more precise, it's 290 g against 294 g. Besides that, the Tab S actually feels a bit lighter to me because the weight is more balanced in that slightly bigger device. I bought the Tab S last week after using the Nexus for a year for its better screen as well as the sd slot and the lack of multitouch issues (which plague all N7 2nd gen IMO more or less). Anyway, I did prefer 7" before the purchase but now I thing the (light weight) 8.4 device is a better compromise all around. The handling comfort is really around the same or for me a bit better on the Tab S and it is just as mobile (fits easily in the back pocket of my baggy jeans).
Yeah, Comixology as well as CBZ/CBR files profit allot from the noticeable size increase (still talking about N7 2k13 against Tab S 8.4). Plus the deep blacks (as well as the vibrant and adjustable colors) help with the large black parts in most comic art and keep the battery usage down.
Interesting points you highlight here. The handling comfort seems to be a matter of personal preference. As far as I am concerned, I use the Nexus 7 without cover, and hold it in one hand when reading.
The Tab S is the same weight without cover, but I find it less comfortable to hold single handed. Worse, I find the samsung touchkeys to be a permanent annoyance when handling the tab without a cover, because its borders are so thin it's though to find a proper place to grip on it. I therefore use it exclusively with its top grade official cover, which is an awesome stand and improves handling but adds 50% weight and doesn't make it thinner for single handed use.
That and the fact the N7 offers a smoother experience on Moon+ Reader lead me to prefer it for ebooks reading. And I can't help thinking about how good the Huawei X1 with its 220g should be for this usage.
Now, I totally agree that the tab S's size is more versatile option. 8.4" is an excellent compromise that led me to completely give up on 10" tablets. But I'm not much of a comics reader, and don't care about the screen size for movies.