Version 2, out in March, offers modular synthesis...I reckon it will be what youre looking for, in the software realm anyway.
NI's maschine also includes a drum synth, and although it sounds great, it wont offer Sparks under the hood flexibility I think.
Signature sounds are good in Spark, I think a further 50 kits are added to version 2 as well. Layering samples is straightforward, each pad can hold six that can be triggered in various ways, velocity, random, stacked etc. I think the way it handles sample playback and a modular drum synth will give it the edge over the competition.