I'm not sure how to explain the differences between them, I have 3
drinkwells but only one has the additional reservoir on the back of it.
It's just a separate unit that attaches to the back of any drinkwell
fountain.  It might be best for you to get the one with the reservoir with
it (you can always remove it later, and the pet fountain will work fine
without it) and show the pet sitter how to fill the reservoir, but tell her
to only do it when the bottle is empty, or easier still, to not fill the
bottle unless the fountain is making 'spitting' noises - which is what it
does when the water level gets too low.

I have the one with the reservoir.  All the petsitter would need to do is
fill the bottle with water, probably every other day if you only have two
cats.  BTW - your cats will love the fountain!

Drinkwell just has one basic fountain.  The basic unit comes with the
pump & bowl/filter unit.  You can also buy a separate reservoir to
attach to it.  It's pretty much just a plastic jar & platform that
hooks into the back of the basic Drinkwell.  There are two reservoir
sizes, 50 & 70 oz.  I have the 50 oz. attachment at work and at home,
where there are two cats each.  When the basic Drinkwell unit is
filled and the 50 oz. reservoir is filled and attached, it lasts the
two cats about a week.

This is a picture of the basic fountain:
http://www.petco.com/product_info.asp?sku=7956291995&dept_id=%2D2

If you don't get one of the reservoirs, water would have to be added
to the fountain every day or two.

Liz

I got the big one, it hurts more when I throw it at the cat.