Beatstep is a controller and sequencer, it doesn't make any sound.
Spark has 16 channels you can use them as drum sounds or any samples/loops you want, there are 6 sample slots per instrument that can be trigger a different sample based on velocity (plus other modes such as round robin). With V2 You will be able to use it for just 16 modeled mono synths if you so wish using the same synth engine used in
Origin, the point is it doesn't have to be drums, it can be any samples you want, but the workflow is the same kind of intuitive flow you find on a hardware drum machine, but even better.
Just looked at the specs for the sr18 and except for the fact it won't use any CPU and it's standalone hardware, I can't see how it would even begin to touch the power of Spark. I don't mean that as a diss against the Alesis as I'm sure it's a good drum machine, but Spark at a push can produce complete tracks, has 64 patterns per preset that you can switch on the fly. It's a great performance machine, you can record your performance inside your DAW as VST Automation. Not to neglect the 2 insert Effects per channel with a great selection of FX, 2 send effects, and 2 master effects, plus you can sequence all the insert parameters within Spark.
If you don't try the demos you will never know