Thanks for your note. I do actually have lots of experience with different Camelbaks and just got ANOTHER new one yesterday too.
Originally I used older versions of both the Charm and the Hydrobak (both 50oz/1.5L) but the chest strap was an addition I made using my MacGyver skills--added a buckle to the ends of the shoulder straps to clip them together. The chest strap really reduced the pack from bouncing when you are running.
The Annadel I like as well and raced my last 2 IM's with it. It comes with its own chest strap but for some reason there's no the tube trap (http://www.amazon.com/CamelBak-Tube-Trap/dp/B002T3KT9S
), which I ordered separately. I ended up moving the chest strap down lower to where my other chest straps were (under the chest where you would put your HRM strap) instead of higher up on the sternum.
My newest one is the 2010 Classic (http://www.camelbak.com/sports-recreation/hydration-packs/2010-classic.aspx
) which is a little bigger at 70oz/2L, but super streamlined and comes with its own chest strap.
They all have pros & cons...what I like about the Annadel is that it's women's specific so the straps seem to fit better. There is also some extra room for storage which is great for long training runs, where you can put some snacks in. For racing though I would probably use something more streamlined like the new Classic, and maybe not necessarily fill up the entire 2 liters (I usually have 2 x 1.5L Camelbaks I switch out in the middle of the marathon).
The Charm is the women's version of the Hydrobak...what I would suggest for you is that yes, 1.5L is probably a good size for half marathon training, probably get the Charm vs. Hydrobak and fashion your own chest strap, or get the Annadel and make sure you order a tube trap too!
Hope that helps! Let me know if you have any other questions