I use Live too and I own a Spark.
The first thing is that you totally need to forget the Live Drum Rack, and honestly we need to say that's really pretty and simple to use.
The difference between the Drum Rack and Spark, IMHO, is that the Spark is more "deep", so it will take more time to make a loop on the fly, internal beats are nice and you can use your samples too with the drag & drop feature.
There is something I don't like: I can't record a beat in Live using Spark Pads, I don't know why, that's weird.
If you just want a Step Sequencer, I don't think you need to spend all that money for the Spark, there are lot of Step Sequencers VSTs out there for lower price, while if you want to have a Drum Machine feeling, Spark is what you need.