Use the same volume for both quotes
For a 10 × 10 ft slab at 4 in, the base volume is 33.33 ft³. With 10% waste it is 36.67 ft³, or 1.36 yd³. That is the number to compare with a ready-mix supplier's minimum order and delivery terms.
The 80 lb product reference lists 0.60 ft³ per bag, so the same adjusted volume rounds to 62 bags. Smaller 40 lb bags list 0.30 ft³ per bag and would require 123 bags for this example.
Count labor as part of the decision
Bagged concrete requires mixing, moving, placing, and finishing each batch. A truck can reduce batch handling, but access, discharge time, site preparation, and the supplier's minimum order matter more than a generic bag-versus-truck rule.
For fence posts or a small repair, the post-hole calculator and bag table are usually the useful path. For a continuous pour, price the adjusted cubic yards and confirm the delivery window before you schedule the pour.