I have a 4 qt and a 6 qt. I use them for different things- we are a family of 6. To cook a whole ham I need the 6 qt. Applesauce, rice pudding, baked beans, or warmed drinks need the 4 qt. Generally I would say that main dishes need large capacity cookers, side dishes and beverages need smaller ones.. (and no you can't just get a big cooker and fill it to a small quantity- it doesn't work that way).
Personally I wish I had bought both of them with the snap on lids and built in timers. My 6 qt has the snap on lid hooks and it is GREAT for taking to potlucks and such.. I wish the smaller one had it. Also, my bigger one I can actually REMOVE the crockery to clean it, but my smaller one it is attached, so I have to sort of scrub it while rotating and let it air dry.. the removable one is much more practical for cleaning purposes. But then there was a big price difference between the two and I probably got them 10 years apart, also..
There was one that sears was selling that was ONE cooking element but came with three different sized crocks. I think that would be incredibly useful. (I just checked their website, but I didn't see it- I know they were in my local sears around Christmas, though)http://kitchen-dining.hsn.com/hamilt...264437_xp.aspx
here is basically what I saw, unfortunately I don't think it has a timer or snap on lid.. although if you don't need it for potlucks, maybe the transpo thing wouldn't be a worry for you... and you CAN buy a timer separately.. it plugs into the wall socket and then the crockpot plugs into the timer.. (I think? I can't find one here, so I am not certain).
good housekeeping provided this- http://www.goodhousekeeping.com/prod...d-slow-cookers
which included an evaluation of customer help lines.. (I hadn't thought of that!)