The S model is the new one with SAFE technology. It's a much much much better plane for beginners. If you're already a competent pilot then get the cheaper one. If you ever wanted to use it for training new pilots then you'd be glad you had the new S model.
got it, i have a firebird stratos but im having problems getting it to fly straight (i think im getting unequal thrust from the engines), so im in the market for a new plane and i think im gonna go with this one.