Chairman,
I have used both a Platy and the plastic bottles. Both have their advantages and disadvantages.
My hydration system allows me to get water without having to go digging through my pack to get to it or even having to take it off to get to those tight spots, so whenever I want a drink, I have a plastic tube right beside me. My pack even has a special pocket for the hydration system. However, trying to get water out without spilling a great deal can be difficult at times.
Small Bottles can fit in side pockets, but usually anything more than a liter bottle forces you to carry water on the inside of the pack, forcing you to stop and go rooting around to get it. I know that on my last hike I used every sip of water in my 2-liter Platy and all in my Nalgene in one day of hiking, so I don’t really recommend two or three 20 oz bottles. You could get very dehydrated if not carrying enough water, which of course we all know can cause serious health problems.
Furthermore, “reliable” water sources may not always be abundant. An example is the AT in New Jersey where reliable water sources are a long ways in between. I would hate the thought that I would have to get by with only a couple of Pepsi or Coke bottles of water for an entire day carrying a full pack.
Additionally, if it has been even a moderately dry season wherever you are hiking, you are less likely to have reliable water sources than you would say at a different time of year. Case in point: the Southeast right now.
All in all, go with the hydration system (I recommend Platypus) and use the plastic bottles for other items like alcohol for an alcohol stove.
Trekking Trav