I've been informed by many different sources that all diesel fuel in the Portland Metro area comes from the same supplier on Swan Island. Your best bet is to get it from the station with the lowest price. Who cares if the station is dirty? You can't pump your own fuel anyway.
Check out
http://www.gaspricewatch.com/USGas_index.asp for the lowest price in your area.
It's not necessarily cheaper across the river in Washington anymore!