Al Downie

I have one of those Topeak large bottle cages kicking around in the shed, and am thinking about putting it on the bike for the forthcoming tour in Ladakh. There's enough room for either two normal cages with my SIGG aluminium 750ml bottles, or the large cage with a single 1.5 litre plastic mineral water bottle. I'm leaning towards the latter, a single large bottle, because that does leave some room in the frame for my pump and a small frame bag full of tools, but I expect I'm missing an important reason to carry two bottles rather than one. (I'll have a few Platypus bags in my panniers)

What would your preference be?

xilios

It just comes down to preference. I have 3 small bottles and my wife carries 1 large (pics on our page) we both like what we have and would switch if need be.
If you need room for the pump go with the 1 large.

neilfein

Multiple small bottles allow you to take a few extra and pack them in your bags. Even if carrying 4 or 5 bottles is overkill, you can always store stuff in them until needed. (Say, when you hit a long stretch with no towns or water.)

Boudicca

Two (or more than two). Means you can always keep one with pure water (to pour over head, wash out wounds, clean hands) and one with something more useful for hot riding days (including fruit juice, and perhaps a little salt and sugar if you are not in Gatorade territory). Also the plastic off the shelf bottles break, crack and otherwise get destroyed, while you can always use the sigg ones as hot water bottles if it's cold at night.

Machka 

+1

Boudicca said exactly what I was going to say.

Plus, the bottle reserved for pure water can also be used at the campsite to collect water for cooking purposes.

Camel

Go with the 2 smaller cages IF you can put one cage on the downtube, and one on the seat tube.

The small cages can hold 1.5L bottles (depends on frame size). You may have to squeeze them a bit to fit, and use velcro straps to keep the bottles safely attached to your bike while riding.