Seems like SX has only 2 trig inputs so the choice is quite simple: SPD-30. There's from time to time used ones om <Craigslist and similar.
I thought of that anyway from the start cus it has probably better feel to the pads and Moreover the SX is more for sampling and trigging loops. Dunno if you're in to that. You can always play drum libraries via USB from computer. Even better sounds. Good luck!