While the best soda is from a can directly into an icy glass, and 2nd would be a coke from Chick-Fil-A cause they use lots of ice and a styrofoam cup. Any other fountain drink just never really turns out well, it either doesn't taste right from the start, or gets watered down before I can finish it. Plus it's a hassle to carryout a fountain drink (drive through fine, but not from a carryout place).
The 20oz bottle is perfect for me, as it'll normally last all day, I occasionally put it back in the fridge at work to chill it back down, but am fine with it either way. It's great to be able to put the lid back on so doesn't get spilled, and it doesn't sweat on the counter, and i don't have to worry about throwing the ice water out in the sink before throwing away, just straight in the trash.
I don't really mind the 16oz bottle, as I figure it's saving me calories, as I don't go get a 2nd one. My mom always taught me to finish my plate (or glass) so anything that psychologically will get me to consume less is good. Plus at a convienence store I'd rather pay $0.99 for the 16oz than $1.59 for the 20oz. Here the 20oz is still more popular in the grocery stores, just coke/dietcoke in 16oz. (we have mostly food lion and wal*mart here).
Anything to keep me from chugging 12 oz cans. By taking a few sips from a bottle, then putting away and coming back helps. I found myself drinking about 5 cans a day, because once it got 1/2 empty, i'd chug the rest to empty the can to toss in recycling bin. Didn't want it to sit around and get flat.