I purchased a HK-55 about a month ago and have had an interesting time comparing it to my KSS-400 and KSS-300. Overall, I think the Mafell saws feel more solidly built, and the Mafell tracks are superior to the Festool's.
On the other hand, the HK-55 has some nice creature comforts, such as the rear mounted bevel knob (with front and rear locking), thumb activated blade guard control (a la KSS-80, KSS-300, etc....), variable speed, and integrated Plug-It.
Ultimately, for me what it boils down to is my perception that the KSS-300 and KSS-400 each do something specific very well, whereas the HK-55 seems a little more versatile, serving well in a role as a generalist but perhaps falling a little short in the niches that KSS-300 and KSS-400 have carved out.
For instance, the KSS-300 and HK-55 both fit nicely in a Systainer 4 (the HK requires a separate bag for the FSK/FS tracks), thus making for an ideal "daily driver" saw that I can always keep in the van. The HK-55, fitted with the FSK-250 track, has a similar crosscutting capacity to the KSS-300, but greater depth of cut, whereas the KSS-300 is vastly more nimble and lighter.
On the other hand, fitted with the FSK-420 track the HK-55 performs similarly to the KSS-400. The HK-55 integrates more easily into the rest of my tools (mostly Festool), but the KSS-400 just seems a litter tougher, and I think the KSS-400 handles just a little more nicely.
The above being said (the KSS saws perform better at their niche jobs better than the HK-55), the HK-55 costs roughly 1/2 the cost of either KSS saw, and effectively performs the work of both at 1/4 the overall cost of two KSS saws.
Pros of the HK-55:
-- Variable speed, should be handy for cutting steel, aluminum, plastics, etc....
-- Plug-It cord makes it easier to swap with my other tools
-- Thumb actuated blade guard retraction
-- Accessories are readily available at local retailers
-- 3 year warranty with clear procedure for repairs
-- Fitted with FSK-250 it is lighter and more nimble than KSS-400
-- Greater power and depth of cut than KSS-300, with greater crosscutting capacity using the other FSK tracks
-- I can buy two HK-55 saws for the same cost as a single KSS saw
-- Greater range of accessories for the Festool FS tracks (parallel guides, TSO GRS-16 square, etc.....)
-- Depth stop functionality
Pros of KSS-300/KSS-400:
-- These saws feel like they are of a higher build quality
-- Mafell tracks are easier to join
-- Mafell tracks are more compact and easier to transport
-- KSS-300 is more compact than HK-55, giving it the edge if I was only using it for fitting and mouldings
-- I prefer the dual column, lever actuated plunge action of the Mafell saws
Overall, If I only did one task consistently (building decks, running millwork, etc.....) then I would hold on to the KSS saws. However, my career seems to be taking me in more of a generalist direction, where it is more advantageous for me to have more of a generalist saw. I still haven't made up my mind on this, but this represents where my thought process is taking me.....