Is there a difference between seed-starting potting soil and sterile soilless mix?

The answer to your question depends on what the ingredients are. Usually a seed-starting mix is also soilless and thus relatively sterile. Seed-starting mix usually has finer particles and may have slightly different ingredients, but peat-based soilless mixes with vermiculite and perlite are an option. When you’re a beginner, a seed-specific mix is probably your best choice.