I presume you are in the US.
PRO's in the US do not pay for theatrical (ie: shown in the theater) works. Not sure how this applies to trailers. My guess would be that they will not be counted if shown in theaters.
On Television though, you may get some payments. Generally I'd think of these more as promo's than a trailer, although I suppose it actually IS a trailer, not a promo. If you music survives the cut down to :30 seconds and onto TV, generally speaking, ASCAP is better at paying out for promo's, but BMI is catching up. BMI is traditionally better for underscore. Why not ask the company where THEY think you should sign. They obviously have their (an by default your) best interest in mind.
HOWEVER, unless you only plan to write trailers the rest of your life, you're much better off taking a look at the BIG picture. What are you going to be doing down the road. Songwriting - ASCAP. Music for TV - BMI. Or ???