osmosis

Recently a few of us biology teachers at SFUSD had an interesting discussion about osmosis while reviewing texts.  I'd be curious to know how you out there explain osmosis.  In our discussion, some of us and some texts emphasize that the rate of osmosis is is driven by  the dffusion gradient of water.  The potential energy of water can be calculated from the concentration of total solutes (osmolarity), so osmosis is a colligative property,  not dependent on specific properties of different solutes.  Others emphasize that ions and polar molecules attract water thereby reducing the energy of water.  This causes a reduction in the activity of unbound (free) water.  My problem.  I think both are right but I don't like emphasizing the latter explanation because this doesn't explain how  osmosis is a colligative property.  I would think that a larger solute molecule would take up more space and attract more water molecules than a smaller solute molecule, but apparently not.  So my two questions are:  1)How is osmosis a colligative property given that different solutes must take up different amounts of space and attract different amounts of water molecules? and 2) How should I teach this stuff to biology students?