Good stuff. Thanks for the input so far. Perhaps a little more info to muddy the water (yes, pun intended, sorry). The reasons I lean toward the GI canteen are:
1) Convenient size for day hikes
2) The carry pouch has extra pockets for water purification tablets and other accessories.
3) The canteen cup nests in the bottom with no additional space requirements.
4) A canteen cup stove can be added as well; it also nests so no additional space requirements.

The reasons I lean toward Nalgene (or possibly other bottles):
1) About the same capacity as the canteen.
2) A carry pouch with extra accessory pockets is available
3) A metal cup is available that nests in the bottom
4) Measurement markings on the bottle (mostly for cooking reasons, but also to monitor hydration if needed)
5) Compatibility with filtering systems
6) Compatibility with scouting

The reasons I lean toward a hydration bladder:
1) Constant hydration via drinking tube and bite valve
2) Carry in a pack on your back
3) Good or better capacity

My concerns with the canteen:
1) Small neck and can't see the nasties inside (if there was any funky stuff growing in there I may not know it)
2) Incompatibility with filtering systems (although I believe several of you have mentioned this is not something I should be worried about)
3) Incompatibility with scouting
4) No measurement marks

My concerns with the nalgene:
1) No nesting stove (although I'm fairly certain a small stove wouldn't be too much trouble to take along)

My concerns with the hydration bladder:
1) Difficulties cleaning
2) Difficulties removing water for cooking or other uses
3) No measurement markings
4) No cup or stove unless I pack it along
5) Compatibility with other backpacks
6) Filtering compatibility
7) Scouting compatibility

For the record I already have two GI canteens and a bladder with its own pack (no other room except for the bladder but does have molle attachments). I bought it on e-bay for a song and dance but I am not really sure how compatible it is with other back packs as I mentioned above. Do I remove the bladder itself and place it in another backpack or leave it in it's current pack and attach it to my main pack or wear my main pack over top of the water bladder pack(I really don't think so on the last as this seems really uncomfortable)? I am more familiar and comfortable with canteens or bottles than the hydration bladders but perhaps with time I could learn. I just don't want to invest any more money without some input. Based on this comparison the nalgene appears to be a better choice but I am still interested in what everyone else has to say. Also I know there are other threads on the subject but as I suspected there seems to be some new products and new opinions. It may seem a mundane topic but no one will question the importance of hydration. Based on my observations here and the original premise of the thread does anyone have any other insights? I appreciate everyone who has taken/is taking the time to contribute.