I think it would be best to start with stainless steel can -- it is much easier to clean it after a coffee enema.
Alternatively, you can start with a plastic can: it is cheap and easy to clean too.
In my view the rubber bags are good for tap water/saline water/soapsuds enemas as those fluids do not leave any "plaque" on the rubber (which would be very hard to remove from the inner side of a rubber bag. Nevertheless, if you get like enemas you should invest to a rubber bag, I think, since while cans can be cleaned much easier, their maximum capacity is 2 liter which is not so much for a good cleansing enema...
Rubber bags can be purchased with even 4 liter capacity.