It's kind of like comparing a horse to a camel. So many differences, yet you can put a saddle on either one. Some people want to ride the horse... some people want to try the camel out instead. Each animal is distinctly different and superior under certain conditions.
(okay, that was a pretty bad analogy...
The Sten is heavier (once you factor in the battery pack), but at least its weight is evenly distributed.
The Sten has a smaller head unit (because Spark puts their battery in the same housing).
The Sten (premium) runs longer on a similar setting to the Spark IF you own the larger battery pack.
The Spark is much brighter (on paper anyway).
The Sten has the ability to change out the optics for different beam profiles (all you can do with the Spark is go from spot to flood).
You can change out batteries with the Spark (you have to have a second pack for a Sten or an alternative power source pack).
In theory, the Sten is probably more waterproof (although both lights should do just fine in almost any caving scenario).
The Spark does not have an exposed cord to worry about.
(There are MANY other fine details)
If you prefer a Sten, are you willing to pay 3 times as much for it? If you prefer a Spark, you will save more money and have a brighter light that is also more "fuel efficient," but are you willing to do without some of the Stenlight's superior characteristics (briefly mentioned above) and longstanding good reputation?