FIT
The bucket design of the 560M is an ergo design, it's a narrower fit for smaller hips but it's tapered to also accommodate larger hips. I had to pad the Spark, I do not have to pad my 560M. My wife couldn't even get into the Spark, but can fit the bucket of the 560M nicely.
Like most shorter paddlers, the hump of the Spark was totally getting in my way, but the hump is nearly non-existant on the 560M.
HANDLING
Anyone's who has been on a Spark Downwind will tell you that it surfs really well - and I concur. The 560M accelerates faster, and responds even better to direction changes. I enjoy the 560M even more in all DW conditions.
SPEED
Flat water speed was the easiest to compare: The 560M is amazingly faster: I don't feel bored, and it doesn't feel like I am pushing water. The Spark is quite fun DW, but a "drag" for training on flat water. It's rocker doesn't help there.
Acceleration was pretty easy to compare as well between the two: as soon as I compared accelerations on the 560M against my usual paddling buddies, the difference was obvious. The GPS started showing numbers I had never seen before. I care very much about acceleration, especially in DW conditions where seemingly, that is ALL I DO.
Downwind speed is more tricky to compare, but I have ample data, enough to clearly say:
The 560M is more agile, requires less effort to link waves and to accelerate: hands down, it's faster.
You'll be "drier" in the Spark, but it will take much longer to drain its cockpit. You'll get more wet in the 560M but the cockpit will dry before you can look at it again.
Moving your body fore and aft will affect the 560M. Moving my 57 kilos forward or back didn’t do much to the Spark. Downwind, I save tons of paddling energy by simply leaning my body on the 560M.
ODD STATEMENTS
The 560M is even more fun. Don't get me wrong: if you were to remove the 560M from available skis today, you'll probably agree with me that the using a Spark DW is one of the most fun you will have. If you can handle the 560M, you won't be looking for another ski.
As a side note, I actually find the 560M more stable DW - like it's on rails.
I used to say that the 560M was a clear choice for paddlers who were 75 kilos or less. I take that back, because it’s more tricky than that. Now, and from having place several paddlers in the 560M, I will recommend the 560M to paddlers that are specifically:
Less than 70 kilos for rough water.
Less than 75 kilos for calm water (including flat water).
Short or long legs.
Narrow hips, or a little wider.
AND…
Paddlers who do not mind being part of the environment
Who are willing to use their bodies, instead of just “sitting there…”
Paddlers looking for a ski that doesn’t toss them around with side wind.
SHORT VERSION
They are two super fun skis - the Spark is one super fun ski in Open Ocean as I find it more agile than most skis, but the 560M is even more responsive due to its size.
With its much deeper bucket, the Spark is a little more stable.
The 560M is still more stable than the V14.
The 560M is pretty much in a league of its own, and very well suited for us smaller humans: more fun on Downwinds, and faster on flat water.