I am hoping that Yamaha do release details on a mid price point kit, similar price point to TD9 ish, as the current DTXpress not quite upto it, in my view.
TD9 (not SX - $1499.97@Sweetwater lists $1999.00):
2 zone snare
3 x 2 zone toms
kick
2 zone hihat
2 zone ride
2 zone crash
+
additional zone for ride
2 zone crash
2 zone aux
= 15 trigger zones (+ additional 5 with expansion) = 20 trigger zones
DTXpress IV Special ($1299.99@Sweetwater lists $1949.99):
3 zone snare
3 x single zone toms
kick
2 zone hihat
3 zone crash
single zone crash
3 zone ride
+
3 aux trigger inputs
= 16 (+ 3 pads with expansion) = 19 trigger zones
It's really a matter of taste whether you prefer the Yamaha sampling approach over the Roland Sampling/COSM approach, and then whether you like having Roland's entry level mesh snare over Yamaha's top end gum rubber snare. Yamaha cymbals are an easy win IMO, and while you can update the TD9 to a VH11 at considerable expense the XPress IV has RHH135 out of the box.
I think they come fairly close to eachother and it comes mostly down to preference. The all mesh TD9 would win hands down - but that's another US$1200 and you still don't have the VH11.