A few comments:
Any hydration system should fit easily and cleanly to a water filter. Try attaching the tube from the bag directly to the out port of the filter. It's by far the easiest way to get the agua in.
32 oz is a little small. If you go with the 70, you might drink more water. If it seems a little too big, you can always fill it only partway full....but you probably won't.
I also like the BigZip Platypus bags for their ease of cleaning, but have learned the hard way that you're better off never opening them in the backcountry because they are a pain to get closed again. The bite valve on this unit is prone to leaking, so be careful not to set anything on top of it. Even so, I like both of the ones I have.
I have a Platypus Roadrunner that I have when all I want to carry is the water bladder and maybe a sammich. It's a nice little piece, and I use it way more than I thought I would.
Once you use one of these things, you will wonder how you ever lived without one. At least that's what I found.