About 25 years ago I used to put up sheds for a living- from 2-car garages up to large sawmills & farm sheds, largest we did was 154' clear span. There are certainly more than two ways that you can do the bases but I would not do either of the methods you have mentioned. As others have mentioned, poles set in the concrete will rust around the bottom- I have this problem at my property with a lean-to wood shed the previous owner put on the back of my main shed. Here's how I would do it;
Make your columns (poles, uprights, whatever you like to call them) with a square plate welded to the bottom with four holes at the corners. Using one of the baseplates as a template, weld up a cage for each footing that will have four threaded rods sticking up out of the concrete. For your size building, some 9/16" all-thread would be plenty strong, drill your base plates 5/8" so they slip over the threads easily. The welded cages serve two purposes- they hold the threaded rods at the right spacing, plus they set in the concrete so that you don't just have straight rods that will pull out.
After digging your foundation holes, set up a pair of hurdles at each hole and suspend your foundation cages in the holes by their nuts, set them up perfectly straight, square and level. (we used to have fabricated jigs to adjust and hold everything in position, but you could suspend a plank with four holes across the hurdles to hang your cages). Grease the threads so the concrete won't stick to them.
Pour your concrete around them and trowel off to form a nice flat pad. It is not critical to get the top of the concrete footings perfectly level at this stage but getting it as close as you can will save work later.
Make up some sheet-metal (I'd use 2mm thick, don't know what the imperial equivalent is) plates about 2" square with a 'dot' in the centre, the 'dot' is just a small round piece of metal about 1/2" thick- something like a 3/8 hex nut would work, or ask your steel supplier for some slugs from their hole punch, about 1/2" thick by 1/2 to 3/4' diameter, whatever holes they have been punching out. Tack weld the dot in the centre of your 2" squares- one for each column.
Once the concrete has set, place one of your 2" squares, with the dot up, in the center of each pad, then go around with a laser level or dumpy level and get them exactly level. Use sheet metal shims under the dot plates to bring them all up to exactly level with whichever one starts off the highest.
You can now place your columns over the threaded rods, the base plate will pivot on the 'dot' and you use the base nuts to adjust the column to vertical. After your frame is complete, grout under the plates with a slurry of cement.
This may sound like a lot of effort but if you use this method and get everything square, straight and level (I used to work to less than a millimeter tolerance in 3 dimensions, even with a shed 100' x 150') and work to similar tolerances with making your frame members, the shed will practically build itself from that point on.
As mentioned, I have used this method for foundations for buildings from double carports up to sheds you can drive a semi-trailer in circles inside. It will enable you to stand your columns up and assemble the rest of the frame with ease.
