I guess it depends on how comfortable you are with on-line transactions. I never use a travel agent and would only ever consider it for something like large group bookings. But, I live on-line (work, banking, shopping), others apparently prefer human contact

I often wonder how this industry survives, but I guess there are still people who use them. If that's where your comfort level lies, then use an agent. I don't know if there is any difference in the price on any given day. However, prices fluctuate constantly, so if you checked on-line several days after you booked, you are likely to see a different price.