I would like to build a 12'x16' workshop in my back yard. All the plans I see call for 8x8 members ( see here ), except for one 10'x14' shed that uses 6x6's ( see here ). My walls will only be 8' high or less. The roof will have 6 in 12 pitch. The 6x6's will be cheaper, easier to buy, etc..Please help!

p.s. the 10x14 uses pressure-treated timbers. What are the pros and cons of that?