I'm not sure about the sweet trumpet, but the PSR550 has the sweet flute (my favorite), and some extra guitar voices. I also owned the PSR540 and upgraded to the PSR550. The piano and organ voices are the same, the speaker system is the same as well. The PSR550 has more styles. IMO the acoustic piano doesn't sound great out of the box, had to add some reverb and chorus for my liking and the sustain button must be on (your mileage may vary).
Starkeeper