I think only the light and extra light extracts are so for base malts, while the others are some mixture of base malt and crystal malts. So I would prefer to go with light/extra light so I can have control over the amount and type of specialty malts.
6-6.5lbs of extract for the average 1.04X beer. I prefer plain light DME in the boil for hop additions for pale ales. LME in at flame out to keep it clean tasting & lighter colored. Plain Extra light for my hybrid lagers or other light colored beers.