ATM is the way to go provided you are sure you will not get hit with any fees, and can be assured of getting a good rate. Occasionally I use an ATM, I think I get 30 free transactions per month, and I get a slightly better rate than the best wholesale currency exchange shop.

How much money are you planning on exchanging? One-hundred U.S. Dollar bills get the best rate (vs. 50/20/10/5/1), so maybe cash one at the airport, just to get local currency for first-day incidentals, then change more in town. I'm surprised, but maybe not too much, that SCB and TMB offer different exchange rates at the airport vs. other locations. I guess I wrongly assumed a bank offered the same rate at any outlet throughout the country.

Once in town, wholesale shops like Superrich (Ratchadamri Road, across from Central World) and Vasu (Sukhumvit Soi 7/1: Nana BTS) offer the best rates. These shops are crowded with Thai, Asian and Japanese customers. Obviously I wouldn't spend 100 THB in transportation costs in order to get an extra 50 THB, but for large transactions the wholesale shops are advantageous.