A 55 gallon barrel would keep a lower temp than the smaller ones if kept full. I haven't gone to that size just because I know our well water is high in coliforms and that much water sitting around worries me.
Drop in a UV sterilizer, it will keep the coliforms, algae at bay... Put a float valve on the barrel and you have unattended filling from a hose...
Not brand recommendation just examples...
http://www.ebay.com/itm/KEDSUM-13-S...918373?hash=item2cab80fd25:g:GG0AAOSw~oFXHkAW
http://www.ebay.com/itm/Short-Aquar...hash=item2caae7c807:m:mShLcjdxjanhakwasbPHm_w