Depends on the mission requirement. I only mine when necessary, usually to satisfy some Engineer request or another. One Engineer required CMDRs to refine 500 tons of ore . . . any ore . . . so in that case strip-mining was the answer using just collector limpets. In another case an Engineer required 50 tons of Painite. I was not going to refine several hundreds or thousands of tons of relatively worthless ores to find 50 tons of Painite so selective mining, using prospector (and collector) limpets was indicated. BTW, ignore asteroid fields, IMO planetary rings are much more productive and remember, prospector limpets double your ore yield.